The Homing Instinct: Meaning and Mystery in Animal Migration
The Homing Instinct: Meaning and Mystery in Animal Migration
Bernd Heinrich
The story and science of how animals find their way home.Home is the place we long for most, when we feel we have travelled too far, for too long. Since boyhood, acclaimed scientist and author Bernd Heinrich has returned every year to a beloved patch of woods in his native western Maine. But while it’s the pull of nostalgia that informs our desire to go back, what is it that drives the homing instinct in animals?Heinrich explores the fascinating science behind the mysteries of animal migration: how geese imprint true visual landscape memory over impossible distances; how the subtlest of scent trails are used by many creatures, from fish to insects to amphibians, to pinpoint their home; and how the tiniest of songbirds are equipped for solar and magnetic orienteering over vast distances. Most movingly, Heinrich chronicles the spring return of a pair of sandhill cranes to their pond in the Alaska tundra. With his marvellously evocative prose, Heinrich portrays the psychological state of the newly arrived birds, articulating just what their yearly return truly means, to the birds and to those fortunate enough to witness this transcendently beautiful ritual.The Homing Instinct is an enchanting study of this phenomenon of the natural world, reminding us that to discount our own feelings toward home is to ignore biology itself.
Meaning and Mystery in Animal Migration
Copyright (#ulink_52c913de-cb40-588b-a976-50fc58834d1c)
William Collins
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This eBook first published in Great Britain by William Collins in 2014.
First published in hardback in the US by Houghton Mifflin Harcourt 2014.
Text and illustrations copyright © Bernd Heinrich 2014
Cover photograph © Antagain / Getty Images
The author asserts his moral right to be identified as the author of this work.
A catalogue record for this book is available from the British Library.
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Source ISBN: 9780007594054
Ebook Edition © August 2014 ISBN: 9780007594061
Version: 2015-04-22
CONTENTS
Cover (#u86f17ac5-4aa7-5b07-bd70-520675b83c34)
Title Page (#ud15a4144-4a40-57e6-b598-73c42722fb35)
Copyright (#u33fc8821-b6c5-531d-a274-c3657b95eb72)
Preface (#u6377da0a-6478-5424-bb26-dd7bb85e3c33)
Introduction (#u33d43583-b8fa-58b8-b37d-f6f34df7cf87)
I. HOMING (#ubc69464a-d9e2-59db-be05-270a8cd4216b)
Cranes Coming Home (#u7c1fd44f-e3e8-53a0-9bec-ff448a1fb98d)
Beelining (#ue6751a10-a6c7-5cc2-98f6-31947ad20b26)
Getting to a Good Place (#uf3a156a3-dd3f-5465-b73e-02249414b001)
By the Sun, Stars, and Magnetic Compass (#uded56845-8313-5c9d-8c48-d515f8a491b5)
Smelling Their Way Home (#litres_trial_promo)
Picking the Spot (#litres_trial_promo)
II. HOME-MAKING AND MAINTAINING (#litres_trial_promo)
Architectures of Home (#litres_trial_promo)
Home-making in Suriname (#litres_trial_promo)
Home Crashers (#litres_trial_promo)
Charlotte II: A Home Within a Home (#litres_trial_promo)
The Communal Home (#litres_trial_promo)
III. HOMING IMPLICATIONS (#litres_trial_promo)
The In and Out of Boundaries (#litres_trial_promo)
Of Trees, Rocks, a Bear, and a Home (#litres_trial_promo)
On Home Ground (#litres_trial_promo)
Fire, Hearth, and Home (#litres_trial_promo)
Homing to the Herd (#litres_trial_promo)
Epilogue (#litres_trial_promo)
Further Reading (#litres_trial_promo)
Index (#litres_trial_promo)
Acknowledgments (#litres_trial_promo)
Books by Bernd Heinrich (#litres_trial_promo)
About the Publisher (#litres_trial_promo)
PREFACE (#ulink_604cb48e-a5b0-59f4-8a6b-87e3b8fb5183)
ABOUT A DECADE AGO I STARTED PULLING TOGETHER BITS and pieces on the “homing” topic and in 2011 had a book manuscript scheduled for publication. I was then living at “camp” in Maine, where I had done my fieldwork on bumblebees for years; lately my work involved feeding ravens with cow carcasses in the winter, and I then got interested in beetles that bury mouse carcasses in the summer. Soon the topic of recycling of animal carcasses of all sorts seemed more urgent than the scheduled book about getting to and living in a particular place. So, I put writing about homing on hold. By the time I again picked up my pencil, it seemed as though everything I thought of or had an interest in had, in one way or another, started to have a bearing on home and homing. In the meantime I had also been confronting personal issues of “homing,” and they seemed to take on increasingly similar forms to what I was reading about in animals.
I had already left my academic position in Vermont and wanted to return home to live in Maine, possibly in the home to which I had bonded strongly as a child. I had planted a row of trees there about thirty-five years ago. Those trees, now huge, brought back many memories related to them. They reminded me of my father, who had liked them, probably because he had strong feelings for a row of chestnut trees that also lined the way to his old home in the old country that he had often talked about. Because I had written a book about my father, and not also one about my sisters or my mother, I had come into the disfavor of both. There had been parent-offspring conflict before my mother died, and then the house stood empty. So then there was also sibling rivalry over the estate. It seemed like being in a real-life situation of the sociobiology theories, in an almost perfect rendition of a naked mole rat colony where one of the family finds a big tuber, and the others claim it as theirs, and then a vocally assertive member establishes herself as the matriarch. I realized then that the difference between what can happen to a human and to a naked mole rat family is mainly one of terminology. This thus provided the topic of home and homing a much wider perspective.
INTRODUCTION (#ulink_ee04a767-712f-5a00-a5ba-0704b3eb0255)
Our passionate preoccupation with the sky, the stars, and a God somewhere in outer space is a homing impulse. We are drawn to where we came from.
— Eric Hoffer
With all things and in all things, we are relatives.
— Native American (Sioux) proverb
I LEANED ON THE SHIP’S RAILING AT THE STERN, A TEN-YEAR-OLD boy with virtually no notion of where my family might be going. I heard the deep roar of the engines, the whine of the wind, and the rush of the churning water. I felt adrift, as though carried along like a leaf in a storm, feeling the rocking, the spray, and the endlessness and power of the waves. I had no notion that we were among multitudes who had made hard decisions to court the great unknown, or any clear idea of why my family had left the only home I’d known in a forest in Germany. The only picture of what our new home might be was that we might find magical hummingbirds, and fierce native tribes armed with knives, bows and arrows, spears, and tomahawks.
Security for me was the memory of where we had come from, specifically a little cabin in the woods and a cozy arbor of green leaves that enclosed me like a cocoon where I could see out but nobody could see in. It meant a feeling of kinship with the tiny brown wren with an upright stubby tail that sang so exuberantly near its snug feather-lined nest of green moss hidden under the upturned roots of a tree in a dark forest. I had in idle moments in my mind inhabited that nest. I found, too, the nest of an equally tiny long-tailed tit. This little bird’s home was almost invisible to the eye because it was camouflaged with lichens that matched those on the thick fork of a tall alder tree where it was placed.
The ocean all around was a spooky void. But then, after several days at sea, a huge white bird with a black back appeared as if out of nowhere, and it followed us closely. I saw its dark expressionless eyes scanning us. It was an albatross. It skimmed close over the waves and sometimes lifted above them, circled back, and then picked up momentum to again skim alongside our boat. It followed us for hours, maybe even days.
The albatross was big and flew without beating its wings. Years later I wondered if, even in the featureless open ocean where so much looked the same every hour and every day, it may have known where it was all along. How do we find our home and recognize it when we find it? These questions were inchoate then, but given the examples of other animals, they put many ideas of home and homing in context.
Later, as a graduate student, I read that pigeons could return home to their loft even when released in unfamiliar territory, and that some other birds could navigate continental distances using the sun and the stars. There were few answers to how they did it. But I read about researchers at Cornell University who attached magnets to the heads of pigeons and got them all confused. Donald Griffin, my scientific hero (who had discovered how bats can snatch silent moths out of the air in a totally dark room that had wires strung all over the place), was releasing seagulls over forest where they could never have been before and then tracking the birds’ flight paths by following them in an airplane. Most of his birds turned in circles before some of them flew straight, although why was not clear. Searching for a thesis problem to work on, I wrote to ask him if birds passing through clouds might keep in a straight line by listening to the calls flocks make while migrating. He replied in a long, thoughtful letter to let me know that this idea was too simplistic, and that one should not discount much more complicated mechanisms. That was excellent advice. I did not then have the means to solve any of these puzzles, but over the years I have kept in touch with the evolving field of animal navigation and its relevance to the need for a home.
For other animals and for us, home is a “nest” where we live, where our young are reared. It is also the surrounding territory that supports us. “Homing” is migrating to and identifying a suitable area for living and reproducing and making it fit our needs, and the orienting and ability to return to our own good place if we are displaced from it. Homing is highly specific for each species, yet similarly relevant to most animals. And the exceptions are illuminated by the rule.
The image of that albatross took on more meaning decades later, after I learned that the species mates for life and returns to the same pinpoint of its home, on some island shore where it was born, perhaps fifteen hundred kilometers distant. During the years when it grows to adulthood it may never be in sight of land. Seven to ten years after having left its home, it returns there to nest. It chooses to go there because of its bond. When a pair eventually have a chick in a nest of their own, each parent may travel over fifteen hundred kilometers of ocean to find a single big meal of squid, and after gathering up a full crop, it then flies home in a direct line; it knows where it is at all times.
The broad topic of homing subsumes many biological disciplines. In order to show the connections among all animals and us, I have interpreted the traditional use of an animal’s “territory,” or “home territory,” simply as “home.” We think of “home” primarily as a dwelling, but in order to be inclusive with other animals, I here consider their dwellings to be their homes as well. My application of the same terms to different species is deliberate for the sake of scientific rigor and objectivity, to acknowledge the continuity between our lives and those of the rest of life. I realize that this smacks to some of anthropomorphism, a pejorative term that has been used for the purpose of separating us from the rest of life. The behaviors involved in homing include drives, emotions, and to some extent also reason.
A home makes many animals’ lives possible: home is life-giving and sought after with a passion to have and hold. We humans are not thinking much about “home” for animals when we confine them in cages devoid of almost everything they need except air, food, and water in a dispenser, or when we destroy the habitat that contains the essentials of home for many species. So I begin our exploration of home and its implications with the example of the common loons, Gavia immer, birds that may live for decades. The collaborative study by three biologists, Walter Piper, Jay Mager, and Charles Walcott, reveals how important home can be — enough for fights to the death.
Loons spend winters in the open ocean, but a pair migrate from it and across the land back to their home, a specific northern pond or lake, to nest along its shore in the spring and raise one or two chicks out on the water. Starting almost immediately after ice-out and almost until freeze-up, camp owners along a lake routinely see “their” pair of loons year after year. It had long been assumed that the same individuals return each year and live as monogamous pairs on their strongly defended home territory. Huge surprises were in store after 1992, when techniques (using a boat, a strong light, and a net) were developed to capture loons and mark them with colored leg bands to identify individuals. In a long-term study of a population of loons in Wisconsin in a cluster of about a hundred lakes, it turned out that a pair of loons indeed returned year after year to their home. However, they were not always the same birds. As expected, given their longevity and reproductive potential, there were many “floaters,” those still without a home, and some of them routinely replaced members of a pair.
The floaters regularly visited different pairs at home at their respective lakes, and spirited vocal meetings resulted. These seldom led to fights, but they were not just friendly visits. These floaters were at first thought to invade others’ home grounds in order to make “extra-pair parenting” attempts (which in males refers to extra-pair copulations and in females to egg dumping into the others’ nests). However, DNA fingerprinting of the young loons from four dozen families produced not a single incident of extra-pair parenting. Instead, the visits by floaters were of an entirely different nature. They had an almost literally “deadly” purpose. The floaters were scouting — making assessments of both the worthiness of the others’ real estate and the defensive capabilities of the resident males — to gauge the possibility for future takeovers.
Loons nest on the ground along shorelines, if islands are not available. Shorelines, if low, are risky nesting places, not only because of potential flooding in early spring, but also because they are within easy reach of raccoons, skunks, and other predators. Most birds test what is a “good home” by direct experience — success in raising a brood there. Or, like the loons, they assess the experience of others: whether or not chicks have been raised there. So, if the territory does yield young, a floater, who apparently finds this out through scouting, may risk a fight in an attempted takeover to remove the defending male (if he achieves takeover, he automatically gets to keep the female who remains on her home). But the floater can risk an attack only when he is about four or five years old. When he is in prime physical condition and has much to gain — namely, a potential lifetime of home ownership — it makes sense to risk the battle. The older territory owner might then fight to the death, presumably because he has much to lose, and almost no chance to gain another home.
Loons may seem extreme in the lengths to which they go to secure a home, as do other birds that risk the hazards of migrating thousands of kilometers. Yet, in the movie I watched with rapt attention on board ship on the way to America at age ten, people on ponies shot arrows at others on a wagon train. All were emotionally charged, because each was fighting for something sacred, and therefore each was willing to risk his life, for defending or wanting a home.
We have learned much about thousands of animal species that twice annually risk their lives to migrate to an exact pinpoint, such as an oceanic island in the case of the albatross or a pond in the vastness of a continent in the loon’s. They open a window with a broad view onto our unending quest into the mysterious minds of animals, and in the process they illuminate our own. How does one tie the vastness that includes other animals, and so much that affects us personally and socially, together into a story?
Writing this book reminded me of when, after riding more or less unconscious in the slipstream of history for over fifty years, I started putting down stones chosen from a vast array of differently shaped fieldstones to build a house foundation. I couldn’t chop them to size or knock the edges off so that they would fit into the inevitable empty spaces to make neat connections. Nor did I want to shape them, like bricks, to make a tidy but artificial structure. The stones found in nature, like facts, are endlessly numerous, wild, and complex. As the famous British geneticist and evolutionary biologist J.B.S. Haldane quipped, “My own suspicion is that the universe is not only queerer than we suppose, but queerer than we can suppose.” I hope to give here a view of some of the “stones” of homing, and their origins, and how they apply in real life.
Homing is central to many aspects of our and other animals’ lives. To understand the meaning of home, like any other phenomenon, it helps to step back and see from another’s world. Animals give more than just clues to the why and the how of homing. They show what is possible, what has been tested, and what has worked over millions of years of evolution.
In this book I cannot hope to provide an in-depth treatment of any issue. My viewpoint is wide, and admittedly personal. Thousands of scientific references are possible for any one topic, and I am not an expert on any topic. Those references I cite are in no way meant to be the specific last word on the topic. They tend to be those I am most familiar with. I apologize for all of the amazing stories and all the details that I have left out. I have tried to speculate freely, and I hope that this will open discussion, not close it.
PART I (#ulink_9a145561-ebcf-5689-ae07-6d273cd92811)
Homing (#ulink_9a145561-ebcf-5689-ae07-6d273cd92811)
A skull I have lying on my desk is as big around as a somewhat flattened coffee mug, but it comes to a point at the front and has two large eye sockets. The bone on the top of the head is sculpted in ridges and furrows and looks like weathered stone. Seen from the back, the skull has a backward-opening hole on each side. The holes anchored the animal’s powerful jaw muscles. Between them is a much smaller hole. I am looking through this hole (foramen magnum) for the brain cavity, but this large head contains scarcely any brain at all, just an extension of the dorsal nerve cord that runs up from the neck.
It’s the skull of a snapping turtle, a female who met her end as she was crossing the road to dig a hole in the gravel to lay her clutch of eggs. As in previous years, she had come from her bog a mere hundred meters away.
The local snapping turtle is no great wanderer. But two sea turtles — the green turtle, Chelonia mydas, and the leatherback, Dermochelys coriacea — are renowned for migrations spanning entire oceans. The leatherback as an adult feeds in northern cold waters but nests on tropical beaches. It can weigh up to a ton, and although the skull of one that I had the opportunity to examine was as big as a basketball, the cavity holding its brain was, like the snapping turtle’s, only a slight expansion at the end of the vertebral column. It could barely have held a walnut. The green’s could have held two hazelnuts. Their skulls hardly differ from that of any turtle, whether of a species alive now or one that lived 215 million years ago, at a time at least three times more distant than that of the last dinosaurs.
The minute dimensions of some animals’ brains are as astounding as the homing capacity of some of their owners. Like albatrosses, sea turtles of various species lay their eggs in colonies with others of their kind on specific ocean beaches. After the young hatch from the eggs buried in the sand, they head for the water and spend years at sea. They may travel thousands of kilometers, and then, a decade or two later when they are ready to lay their eggs, they return to their birthplace. They mate in the water nearby, and the females then come ashore to dig their nest holes in the sand and to drop in their eggs. How are they able to find their old home after years of wandering in the vastness of the oceans, when we, if taken blindfolded to and then released in unfamiliar woods, would, despite our highly sophisticated massive brains, be as likely to head off in a wrong as a right direction? To get around in unknown territory most of us need a map with which to find at least one known fixed feature that we can both see on the ground and locate on that map, and a compass.
What knowledge and what kind of urges does it take for some birds to fly nonstop for nearly ten thousand kilometers, spending all day and night on the wing, until their body weight halves as they not only burn up all of their body’s food stores but even sacrifice muscle, digestive tract, and other entrails — almost everything except their brains?
CRANES COMING HOME (#ulink_ad57f3f7-2050-5a0d-94c3-b1c3f076fd7c)
If feeling fails you, vain will be your course.
— Johann Wolfgang von Goethe, Faust
MILLIE AND ROY ARE A PAIR OF SANDHILL CRANES THAT STAY for most of the year in Texas or Mexico but travel north in April and have for at least fifteen years nested and raised their one or two offspring, known as colts, in a small bog in the Goldstream Valley near Fairbanks, Alaska. Their home is adjacent to the home of my friends George Happ and his wife, Christy Yuncker. George was an insect physiologist and chairman of the Zoology Department at the University of Vermont where I was hired in 1980, and he later moved to the University of Alaska and the land of the Iditarod, where the two built their home in the wild land near Fairbanks. They invited me to visit them and “their” cranes, and I was eager to do it.
The thousands of square kilometers of central Alaska’s permafrost-covered taiga consist of stunted blue-green spruce and white birch, with a groundcover of green-yellow moss and twiggy Labrador tea whose evergreen leaves curl at the edges and have a soft beige fuzz on their undersides. Chalky lichen and small shiny cranberry leaves decorate a thin black soil overlying the permafrost that can extend thirty meters down. In this expanse, there are many bogs or pingos, which are the result of an ice dome (groundwater that freezes into an upwardly bulging ice lens) that has melted and created a depression where a pond or a lake is then formed. After a few centuries, a floating mat of vegetation grows in from the edges to create a floating bog. Such pingo bogs have become the favorite home sites of sandhill cranes.
Portrait of Millie and Roy
George and Christy’s pingo in the Goldstream Valley is, like others, clear of trees but surrounded by stunted black spruces. It is the home site not only for the crane pair but also for Bonaparte’s and mew gulls, pintail and mallard ducks, and sometimes horned grebes. I intended to arrive several days in advance of the cranes’ anticipated return, to try to watch their homecoming. Surely this return during the first week in May would be a big event in their lives, and I wanted to see their reactions to their old home.
Millie and Roy had last been seen as they left the bog in the previous year, on September 11, 2008, for their southward migration. They had been delayed from their normal end-of-August departure date because Oblio, their colt, had a leg injury that prevented him from being ready in time for the family flight to western Texas. Waiting for him saved his life; we know that young cranes, as well as geese and swans, learn the route between wintering and breeding homes from their parents. The proof and the implications of the necessity of the young to be able to follow their parents, or alloparents, in order to migrate were perhaps most convincingly demonstrated by William Lishman after he first played parent to hand-raised geese that he later led as a flock with an ultralight aircraft. He also led sandhills in this way. Finally he led a flock of whooping cranes from their breeding grounds in Wisconsin to establish new homes for them in Florida. However, nobody as far as I know has been able to follow wild birds, and my chances of seeing Millie and Roy touch down for the first time on their arrival this year might be slim. But I felt it was worth a try.
Cranes, like other large birds, grow slowly. It takes them thirty to thirty-two days to incubate their two eggs, and another fifty-five days for their (usually one) colt to be able to fly well enough to migrate. This far north there is only a narrow window of time for cranes to breed successfully, especially for those that fly even farther to breed, as some of those wintering in Mexico do, in Siberia. If late in arriving, they waste their effort of migrating the thousands of kilometers north. If they are too early, snow and ice cover all food sources. This year had been a winter of heavy snows in central Alaska. Even the boreal owls were starving from their inability to reach the voles under the snow. By late April, when I arrived, the woods around Millie and Roy’s home bog were still under at least half a meter of snow, and the cranes had not yet shown up.
I would have liked to fly with the cranes on their homeward journey, but the best I could do, apart from trying to beat them to their destination, was to see a piece of their flight path. My transcontinental flight of 3,872 kilometers from JFK Airport in New York was followed by a direct flight on April 23 from Seattle to Fairbanks, and I spent most of the three and a half hours of the 2,467-kilometer flight from Seattle north in the Boeing 737–800 with my face pressed to the window, trying to see like a crane. How did the cranes navigate and negotiate their five-thousand-kilometer journey from Texas or Mexico to come home to their own pingo out of thousands of others scattered throughout the vast and seemingly unending Alaskan taiga?
The cranes arrive lean at their main staging area, at the Platte River in Nebraska, and stay three weeks to gather reserves for their continuing journey north. When ready, they gather with thousands of others and wheel high in the sky into giant “chimneys,” to travel together on their common journey. Once in Alaska, they take separate paths to their individual homes, and a third of them fly beyond, to their homes in Siberia.
We had scarcely lifted off in Seattle when we passed over white-capped mountains with knife-edged ridges, dark forested valleys, and peninsulas surrounded by blue-gray water. An hour later, cruising at about eight hundred kilometers per hour at eleven thousand meters, there was ever more of the same — white mountains as far as the eye could see. Another hour — it was still the same. To me, barely a feature stood out from the jumble of endless peaks that melded into each other, and the vast mountain scape was broken only by frozen lakes glinting in the evening light. And so it continued for yet another hour. When we started our descent to Fairbanks, I saw oxbows of meandering rivers, and finally the thin thread of a road.
Cranes, swans, and geese travel south in the fall as family groups. On their way, the young learn the route they will later take north in the spring, to come back to try to settle near where they were born. What they see and remember seems astounding. I might, with intense concentration, memorize a tiny portion of the way, perhaps around this or over that mountain. But these cranes come not from my point of departure, the state of Washington, but from considerably farther south. (Four cranes from the Coldstream Valley that the Alaska Department of Fish and Game had equipped with radio transmitters ended up in various parts of Texas in the winter.) I could never retrace even my own much shorter flight route from Seattle, even if I were to return the day after having flown over it, much less a half-year later. What are the cognitive mechanisms that allow the birds to do this?
Day after day for almost eight months now there had been no crane at the pingo. For most of that time the ground had been under a deep layer of snow that locked any food out of reach. What would happen if, after their long-distance flying, the pair were to arrive at their home and find the bog still under snow and ice with no cranberries to be found and no voles to catch? How much can cranes afford to gamble in order to try to come on time, or even early?
It was only in the last week of April, after another snowstorm, that the weather suddenly warmed, and just then, on the 24th of April, on my first morning, we heard a crane in the distance. Still, no cranes landed on the pingo on the 25th, 26th, 27th. But the next morning at dawn I awoke to the loud and penetrating trumpeting calls of a single crane. These metallic sounds are unearthly; as Aldo Leopold wrote in his “Marshland Elegy,” they evoke “wildness incarnate.” On and on this bird shattered the dawn’s stillness, and I ran out to look. But the bird was then distant, and the sound kept shifting position, so I presumed it was flying around in great circles, possibly looking for a patch of cleared ground; the mossy floor of the nearby stunted spruce forest was still covered in deep snow.
That evening we sat down to supper by the window facing the wide-open panorama of the pingo in front of us. The sun was still high. We were just polishing off the last of our freshly grilled salmon when we looked up to see a crane with spread wings gliding down for a landing. Its long thin legs touched down gently on the still-thick ice of the pond. It bugled and sprang up half a meter or so, unfurling its long neck with beak held skyward and with extended wings at the apogee of its graceful leap as if to catch itself in the air to prolong this moment. It looked like a physical embodiment of joy and excitement. The crane kept leaping, all the while continuing its stirring bugling. Cranes don’t do this every time they land; this was indeed a special landing. Finishing its dance, the crane started to walk in a contrastingly slow and deliberate manner, thrusting its head forward and up with each step, and at the same time opening its bill and making a very different, trilling, call.
The crane walked and leaped in several more repetitions of its dance before eventually lifting off with even wing beats to sail off in the same direction from which it had come, its haunting cries growing faint in the distance. Two hours later the (same?) crane came again, but this time it circled the bog only once before leaving, continuing its calling. It had given the impression that it was glad to be back but was at the same time agitated and looking for something; a mate? In previous years Millie and Roy had always returned as a couple, George and Christy told me, so lacking positive identification, we were skeptical that this was either of them.
I had barely gone to bed that night when I heard another one or possibly two cranes calling excitedly, while a third seemed to answer from the distance. I jumped up and rushed outside to look: a pair were walking from one end of the bog to the other. George and Christy were up also, and for the next hour, until it got dark at 11:15 p.m., we watched. One of the cranes stood tall and extended his head and neck forward and up, reminding me of the dominance display of a male raven. This was Roy. The second one, whom I would soon identify by her walk and talk and narrower white face patch, held her neck and head in a more downward curve like a heron’s, projecting her head slightly forward with each step. She started to pick up cattail fronds and grasses and then to deposit them, sitting down briefly on the materials. Was she encouraging her mate by suggesting to him that it was time to start nesting at one or another of these potential nest sites? I could hardly wait to see what would happen next.
Crane pair coming home
Another clamor awoke me in the early dawn, around 4:00 a.m. There had been a heavy overnight frost and the two cranes were standing on the ice in the middle of the bog. Both Millie and Roy tossed their heads up in a quick motion, their bills opening wide during each call. We heard what sounded like a hammer hitting a metal bell or drum, as he opened his bill once to make each call and she chimed in at the same time but called and opened her bill twice to make two short similar cries of a higher pitch. It was a composite call made by both together; a duet. A third, distant crane responded. The distant calls, and the pair’s duet, were repeated back and forth, often and loudly.
The vigor of the pair’s unison calling was still palpable, even when the first morning light lit the sky and silhouetted the black spruces, and when I thought about the enormous effort they had invested to get here, I realized what was at stake: home ownership. The pair’s loud clanging calls attracted no others flying in from the distance; instead, the calls are a vocal “no trespassing” sign, one leaving no doubt that any potential challenger would be facing not just one bird, but a united, cooperating pair.
I watched the pair for another hour. She by then occasionally fluffed herself out and, as she had done the day before, continued periodically to squat where she had pulled at or dropped sedge and other potential nesting material.
The pair continued their slow, deliberate steps that morning while meandering from one end of the bog to the other, as though inspecting every square centimeter of it, and at 9:00 p.m. we saw Roy jump high and with outstretched wings dance by himself out on the ice. Millie dashed by him with fluttering wings, to round out a mutual performance. A little later we heard a purring call as she spread her wings to the sides and stood still. Roy, with outstretched neck and elevated bill, jumped onto her back and, balancing himself with a few wing beats, mated. He dismounted after a couple of seconds. Both then bowed to each other and continued their walk. They were home, intended to nest, and had now sealed the deal.
Still, the lone bird that had tried to intrude on their turf did not easily abandon its intended claim. But why would this crane want a claim without a mate to nest with? Was it Oblio, their grown colt from last summer? Was he, now that Millie and Roy were re-nesting, finally being “thrown out of the nest” by his parents, who no longer tolerated company of any sort? I suspect this was the case. The offspring of most birds have a strong attachment to home. This emotion is a biologically relevant drive, because home is where reproduction has proven to be successful. But the young have a lifetime ahead of them, and if for some reason the parents don’t make it back home, the offspring could inherit their territory. If the parents do come back to reclaim their home, at least nearby territory would be more like the old home than the far-off unknown. Even if a bird is without a mate, finding a suitable territory is often a prerequisite to getting one.
Shortly after we had breakfast the next morning, the lone crane flew over once again, and the pair immediately launched their synchronized duet as a vocal challenge. This time, although the lone crane did not land, the pair jumped into the air and chased it until all three were out of our sight and hearing. But the pair returned soon and then again performed several nest-building probes. They again mated in what would become a routine for the next several days: at least once in the morning and once in late afternoon or at night.
When I first saw the cranes the next dawn, each was standing on one leg with its head tucked into its back feathers. Ice had again solidly covered their pond, after having partially melted along the edges the day before. Both Millie and Roy seemed to be asleep, although he, balancing himself on one leg, occasionally reached up with the other to scratch his chin and head with the toes of the foot. But whenever a crane called from the distance, both their heads shot up instantly, and they renewed their spirited in-unison call, she making the two short notes and her mate making one, at a lower pitch.
We had so far not seen them feed. Indeed, it was hard to imagine that there was food available in any case. If lucky, they might by now catch a vole or find a few of last year’s cranberries, but this year that would probably not be likely until days later when the snow would melt. A crane’s large body size requires much more food than a small bird’s, but that same large size is an advantage in tiding them over during lean periods, and thus to return to their homes before the anticipated flush of food becomes available.
As it got lighter on this dawn, the cranes soon had company. A pair of swans and then a small flock of five Canada geese flew by. A robin sang. By about 7:00 a.m., the crane pair became animated as well, walking over the bog while picking here and there, and again mating.
We did not see the pair most of that afternoon. The lone crane, seemingly to take advantage of their absence, again flew in and this time landed, looked around, and repeatedly made a trilling call. But in about fifteen seconds the pair flew in as if out of nowhere, and one of them sped over the ice as if to attack the interloper, which immediately flew off. The pair gave chase, and all three disappeared from sight. In several minutes the single bird returned. Again the pair came and caught up to the lone crane before it had a chance to take off, and this time they attacked it viciously, in a flurry of flailing wings.
The pair had by now, after the third day, won the major part of the battle. Barring accidents, they would within days lay their two eggs and go on to raise their colt. Normally both eggs hatch, but as in some eagles and vultures, usually only one chick survives, probably because one gets fed less and then weakens and eventually starves. Presumably through evolutionary history, for the fast growth required to reach full development and readiness to migrate by August, there has not been enough food to raise two colts at once. One might suppose the cranes could simply lay only one egg, but sometimes an egg does not hatch, and the second is insurance.
The pair seemed more animated after their last fight with the lone intruder, and by evening they again mated (for at least the third time that day). In most birds one mating is enough to fertilize the eggs. Perhaps several matings are insurance, but this seemed more than enough for insurance. Perhaps, like their dances, mating is additionally part of their bonding ritual.
A pair of mallards, and then a pair of pintails, arrived in the evening, and the ducks swam next to each other near the cranes at the edge of the pond, where some open water had reappeared during the day. The cranes ignored them and again walked in their stately manner back and forth across the ice of the pond, and now they pecked in the low vegetation being exposed along the edges. They were by now finding overwintered cranberries exposed by the melting snow.
On the evening before I would leave for my journey home, Christy and George hosted a potluck party. Shadows fell on the white frozen middle of the pingo as the western sky turned yellow and orange and the spruces became dark silhouettes. A pair of pintails again landed in the open water along the pond’s edge. The cranes were standing, each on one leg, their heads tucked into their back feathers. People crowded around the spotting scope in the living room, watching them occasionally shift position, lower a leg, poke a head out to look around. Suddenly the person then at the scope erupted with an exclamation: “They are mating!” She had seen the male approach the female with her spread wings, mount, flutter, and jump off. The pair had bowed to each other. Suddenly many people crowded around the scope to watch.
Why, I wondered, would anyone, or almost everyone, want to watch cranes mate? Why was nobody interested in watching the mating activity of the two ducks, or of the numerous redpolls? Could it be, I wondered, because we feel a closer kinship with cranes than with other birds?
Cranes are similar to us in many ways. Some are nearly as tall as a person. They walk on two long legs like us, albeit with a much more graceful and deliberate gait, so that they remind one of a caricature of a gentleman or an elegant woman on a leisurely stroll. The sandhill crane’s red bald pate and sharp yellow eye add to the caricature. Cranes form lifelong pairs and stay together as families, but they are also gregarious and join up into large groups. They form a strong attachment to their home. They not only make music with trumpeting calls that sound like bugles, but they also dance, and do so on various occasions.
All of the fourteen species of the world’s cranes dance. Crane dancing involves running, leaping into the air, flapping the wings, turning in circles, stiff-legged walking, bowing, stopping and starting, pirouetting, and even throwing sticks. Dancing is primarily done by pairs and presumably functions in cementing pair bonds and/or synchronizing reproduction. But it can also be induced at any time, and it stimulates other cranes to dance. Even the young colts perform some of the species’ dance. Possibly it serves as practice and could be motivated by the same basic emotions of joy that are an indicator of health important to mating.
Cranes’ dances often stimulate humans to dance as well and have been mimicked in many cultures all over the world where cranes live. Crane dances were performed by ancient Chinese, Japanese, southern African, and Siberian people. If not emulated, cranes are admired. In the Blackfoot tribe of Native Americans of northern Montana, the last name “Running Crane” is common.
Nerissa Russell, an anthropologist, and Kevin McGowan, an ornithologist from Cornell University, revealed that eighty-five hundred years ago at a Neolithic site in what is now Turkey, people probably performed crane dances using crane wings as props that were laced to the arms. Furthermore, someone of these people apparently hid a single crane wing in a narrow space in the wall of a mud-brick house along with other special objects (a cattle horn, goat horns, a dog head, and a stone mace head). Russell and McGowan also found evidence that vultures may have been hunted for their feathers for presumably a much different costume worn as well for a ceremonial purpose. The authors inferred that the cranes were linked with happiness, vitality, fertility, and renewal (since they arrived in the spring). While the crane dance was one of life and birth, and possibly marriage and rebirth, the vulture dance was associated with death and perhaps return to the afterlife.
Russell and McGowan believe that the crane wing interred in the wall of the house was never intended to be seen. It was a symbolic object related to marriage and construction of a new home and may have been coincident with a particular human marriage and home-making. The associations among dancing, pairing, and raising young and home would have been natural for people who saw cranes return to their home ground, just as I had seen Millie and Roy do. Seeing the close parallels in the biology of the birds with their own lives, and understanding the cranes’ dancing as helping to make or cause the good things that followed, Neolithic people would have been compelled to symbolically emulate the crane dance of homecoming and of new life.
BEELINING (#ulink_023ec351-cb60-54c0-8f4c-006a10f6452b)
Observation sets the problem; experiment solves it, always presuming that it can be solved.
— Jean-Henri Fabre
CRANES FLY AN ENORMOUS DISTANCE TWICE ANNUALLY, BUT relative to their size, bees also fly huge distances — up to ten kilometers — and the foragers may perform such trips hourly. We can experiment with them to find out how they navigate. What we know about bee homing so far is nothing less than astounding, and it is built on a long history of research, primarily pioneered by the imaginative experiments dreamed up and performed by an Austrian named Karl von Frisch and his colleagues that date back over a half-century. Arguably, our knowledge dates back still further to early American frontiersmen trying to find bees’ treasure troves of honey.
In 1782, Hector St. John Crèvecoeur, a writer and farmer from Orange County in New York State, wrote:
After I have done sowing, by way of recreation, I prepare for a week’s jaunt in the woods, not to hunt either the deer or the bear, as my neighbors do, but to catch the more harmless bees … I proceed to such woods as one at a distance from any settlements. I carefully examine whether they abound in large trees, if so, I make a small fire on some flat stones, in a convenient place; on the fire I put some wax; close by this fire, on another stove, I drop honey in distinct drops, which I surround with small quantities of vermillion, laid on the stones; and I retire carefully to watch whether any bees appear. If there are any in the neighborhood, I rest assured that the smell of burnt wax will unavoidably attract them; they will find the honey, for they are fond of preying on that which is not their own; and in their approach they will necessarily tinge themselves with some particles of vermillion, which will adhere long to their bodies. I next fix my compass, to find out their course — and, by the assistance of my watch, I observe how long those are returning which are marked with vermillion. Thus possessed of the course, and, in some measure the distance, which I can easily guess at, I follow the first, and seldom fail of coming to the tree where those republics are lodged. I then mark it [presumably with his name to claim ownership].
James Fenimore Cooper, author of the Leatherstocking Tales of the American frontier, of which The Last of the Mohicans is probably best known, in 1848 published the novel The Oak-Openings; or, The Bee-Hunter. Here Cooper depicts a different, perhaps more reliable method than Crèvecoeur’s of the frontier activity that came to be called “beelining.” Cooper’s story takes place during July 1812, in the “unpeopled forest of Michigan,” where, due to the Native Americans’ lighting periodic fires to clear the ground, there were many flowers among the scattered oaks. This was ideal honeybee habitat, and here the bee hunter Benjamin Boden, nicknamed “Ben Buzz,” practices his art. Ben captures a bee from a flower by placing a glass tumbler over it and sliding his hand underneath. He then places the tumbler with the captured bee on a stump next to a piece of filled honeycomb. He puts his hat over the tumbler and the honeycomb so the bee will not be able to escape. He waits as the bee, stumbling around in the dark, eventually finds the honey. Once it is preoccupied with imbibing the honey, it quits buzzing, and the silence is the signal for Ben to remove the hat and then the glass, as the bee will stay to finish its feast and will fly up, circle the honeycomb, and depart directly toward its nest. He then follows the bee to the tree, chops it down, and is rewarded with just over one hundred kilograms of honey. Easier said than done.
American honey hunters eventually added refinements to their beelining techniques. The main improvement was the invention and use of a “bee box,” a small wooden box designed to catch a bee and get it “drunk” on a hunk of honeycomb. It was used in Maine when I was a kid (I still own mine). George Harold Edgell, a lifelong bee tree hunter from New Hampshire, wrote in 1949 in a pamphlet titled The Bee Hunter that “one’s first task is to catch a bee and paint its tail blue” and “this must be done gently [because] bees do not like to be painted. To paint a bee, it is best to wait until it is eagerly sucking up a thick sugar syrup and is too pre-occupied to notice.”
By 1901 Maurice Maeterlinck, the Belgian playwright and Nobel laureate in literature, described in The Life of the Bee his scientific experiments on bees that were individually identified with daubs of paint, from which he deduced that these insects could communicate their discoveries of food bonanzas to hive mates that would then navigate directly to the food. However, American woodsmen not only had used similar methods, but had also, through their beelining, already gleaned that same surprising insight into what the bees could do. Maeterlinck credited his American predecessors for their discoveries and wrote, “The possession of this faculty [to communicate food locations to hive mates that then can navigate to the food] is so well known to American bee hunters that they trade upon it when engaged in searching for nests.”
Although early American woodsmen, whose lives depended almost directly on the knowledge gained by close contact with nature, were beelining devotees who had deduced that honeybees recruit hive mates, it would remain for Karl von Frisch to unravel the marvelous story of how the bees communicate within the hive. He earned the Nobel Prize in Physiology or Medicine for this work. I feel lucky that a Maine neighbor, Floyd Adams, took me beelining when I was eleven years old, and that when I was a teenager, my father gave me an inspiring little book by von Frisch entitled Bees: Their Vision, Chemical Senses, and Language. It explained the experiments that he and colleagues had performed. They were mesmerizing because they connected the practical experience of beelining in the Maine woods with the imaginative power of a scientist who had penetrated into the core of the bees’ world, their hive, their home.
Floyd’s family’s home was the farm four hundred meters down our dirt road. It was populated by chickens, geese, cows, pigs, plus all the other usual and unusual wildlife that lives in a place with a tolerance for disorder. Along with Floyd, my companions were the four Adams boys, Butchy, Billy, Jimmy, and Robert, an in-law of theirs. Floyd, a dark-haired, mustachioed, wounded Marine Corps veteran recently returned from the Pacific, had a bad limp and a thirst for Black Label beer. Leona, his blond, petite wife, appreciated his fondness for honey but less so his taste for beer. He and the “boys,” after a hot day haying, sometimes went fishing on our nearby Pease Pond in the evening, but in August our big draw was always the beelining.
After we found a bee tree, we carved our initials into the bark to proclaim ownership (property lines were irrelevant with regard to bee trees; finders keepers was the rule), and at some convenient time we returned with crosscut saw, axes, wedges, a beehive, and pails and kettles for honey. Getting part of our living from the land was fun, and it meant understanding and using the bees’ homing behavior to find their hollow trees in the forest and resettling them into a new home, which we brought back to the farm and set up at a window in the attic of the house.
Fast-forward to a quarter-century later: My nephew Charlie Sewall and I are in a patch of goldenrod blooming in a pasture where each fall the wild honeybees gather nectar to top off their honey stores for the coming winter. We start by capturing a single bee in our bee box, a simple four-sided wooden box that has a ten-by-fifteen-centimeter piece of honeycomb with sugar syrup filling out the bottom. We dab the box with a drop of anise for scent and capture our bee by holding the box under her after she has landed on a flower and then slapping the box cover over her. At first the captive buzzes in the box trying to escape, but the buzzing stops when she stumbles onto the sugar syrup and starts to tank up, which will take her a minute or two. We then remove the cover and set the open box onto a pole that reaches to just above the tips of the goldenrod. We gently daub her with a spot of paint while she is absorbed in sucking up syrup, as I remembered Floyd doing. We then hunker down into the goldenrod and wait as she continues sucking up her newfound sweets that she will soon share with her hive mates. After about two minutes, her honey stomach is filled. She crawls out onto the edge of the box, stops to wipe her antennae with her front feet, lifts off, and flies back and forth downwind of the box. We duck lower to keep her silhouetted in sight against the sky as she starts flying loops, which become increasingly wider and oriented in one direction. Finally she straightens her flight path and takes off, making a “beeline” into the distance. Knowing that nobody in that direction keeps bees, it’s clear that she is on her way to a bee tree. She will soon be back with others, and we then consult our wristwatches to time her trip. A bee flies about four hundred meters a minute, and it may take her three to six minutes in the hive to regurgitate and unload her honey stomach’s contents into the mouths of begging, receiving bees.
We settle down and wait, and after perhaps ten minutes or less a bee suddenly appears and makes very rapid zigzagging flights just downwind of the box. The sound of her fight has a higher pitch than that of the bees foraging on the nearby goldenrod flowers. This means that she is more motivated and has a higher body temperature because of the rich food she is expecting. She settles into the box and starts imbibing the syrup. More bees will come soon, and when they get near our bee box, they will be guided in by the scent of the anise that marks the spot. After they tank up, we watch their flight directions.
If the food is in the immediate home vicinity, the bee does a “round dance” on the honeycomb when she returns to her home. She repeatedly runs in small circles while shaking her abdomen, and she regurgitates small samples of her find at intervals during her dance. If they become motivated after receiving information about the quality and scent of the food advertised, her hive mates leave the hive and search for the advertised food. If it is beyond a few hundred meters, the bee alters her dance to also contain information concerning location. The distance of the journey to the food is proportional to the duration of the waggle runs, and the angle of the straight runs with respect to the vertical direction informs the bees in what direction to fly when they leave the hive. If the straight run is in the up-direction on the honeycombs (which always hang vertically in the hive), the food source is in the direction toward the sun. If the food location is, for example, at an angle of ten degrees to the right of the vertical, the food direction is ten degrees to the right of the horizontal component of the sun direction when the bee would fly from the hive. Thus, her behavior is a symbolic representation in body movements of the flight to the food.
The first steps in the evolution of recruitment likely involved simple alerting signals in or at the nest entrance before takeoff. Other bees could have followed those signaling bees, probably by scent, for at least a short distance in flight. Through a few million years, the alerting likely became modified to take on an ever-greater leading function by bees flying in an ever more conspicuous manner in the direction of the food, so that followers could start off flying with ever greater accuracy in the right direction. These flights, later in the evolutionary progression, were eventually restricted to a buzz run directly on the top of the combs, but still in the food direction. We can infer this, because such “primitive” recruitment is still found in some tropical honeybee species that have their combs in the open air, where this mechanism makes sense. But open-air homes, though convenient for such communication of food location, were vulnerable to predators and also precluded the bees from living in huge areas of the globe, those with cold climates.
Homes in hollow trees allowed the bees to live in areas where they would otherwise be excluded because of cold and/or nest predators. But in such safer homes the combs hung from the roof of a cavity and left no horizontal dancing platform, and additionally the “dance floor” was now in darkness, so bees could not point directly toward the food. Even if they could, they would not be seen. But a breakthrough for indicating horizontal directions on vertical surfaces became possible after some bees started using the hanging flat surfaces of combs as their dancing platform while indicating the sun’s location as the up or “toward” direction in their dance. Additionally, tactile rather than visual orientation became predominant for recruits in reading the code within the nest.
It is amazing enough for an animal to be able to navigate to a location it has never been to before. But some ants do something even more amazing. In North Africa, desert ants live in underground homes where they are protected from the heat. But they must venture out onto the searing surface periodically to forage by scavenging on heat-killed prey. The ants are fast runners that have evolved a very high tolerance for heat. Still, at times it is a matter of life and death even for them to make it back to their cool underground home; they cannot afford to wander on the sand surface for an extended time without access to their shelter to cool down and replenish body fluids. This is where their homing ability comes in; they may have zigzagged in all directions to find a heat-killed insect, but after finding one they must make a straight “ant line” directly back home. This begs the question, Since they are often on a featureless plain and have not kept a steady course, how do they know in what direction to head home?
If one captures bees in one pasture and releases them in another, they usually depart in the direction they would have flown from the original field. That is, they act as one would expect if they do not realize that they have been moved to a new location. Rüdiger Wehner and his colleagues at the University of Zurich came to the same conclusion about desert ant homing in their lifelong experimental studies. The ants use the sun as a compass, but a compass is not enough; the ants, when released from a point they had not themselves traveled to, like the bees caught in one pasture and released in another, apparently got lost.
For homing you must know where you are on “the map” before you head off in the correct direction. The desert ants can return home, but only if they walk to where they find themselves. Wehner concluded that the ants’ homing mechanism involves somehow calculating where they are at all times, probably in measuring distance by keeping a kind of count of their steps, and also keeping track of the angles of their direction from their home relative to the sun’s location. These were not mere speculations, but a hypothesis tested in painstaking experiments that entailed altering the ants’ perception of the sun (holding filters over them that varied the direction of polarized light that they, like bees, use in orientation) and altering their stride length (altering their leg length by gluing on extensions) to find out what information they valued and how they used it. Presumably bees could also have a similar “map sense,” and Randolf Menzel, a neurobiologist in Berlin, was trying to find out how it might work.
Menzel runs the large and active Institute of Neurobiology at the Free University of Berlin, and one of his projects was the burning question of how honeybees seem to find out where they are in order to be able to go where they want to be. Honeybees are suitable animals with which to study this problem because, like ants, you can count on their motivation to return home after they are loaded with food.
We can’t look into a bee’s brain and determine what it knows and what it wants. However, clever experiments based on the bee’s natural history permit inferences. We can determine, for instance, where a bee perceives herself to be relative to her hive. If a bee regularly visits a feeding place, she knows where she is, because she always flies off in a straight line from it back to her hive. If we then remove either the hive or the feeding spot, she circles in the area where her target had been. We know what she is looking for, because when we provide the hive and/or the feeding station within the area where she circles, she quickly finds it. But suppose we capture our bee at the usual feeding station after she tanks up on honey or syrup, put her into a dark box, and then carry her “blind” to a place she has never been. As mentioned, most bees will then make a beeline in the same direction they had normally flown to return to the hive. They will fly as far as before but find no hive there. Yet, they usually eventually do make it back home. How do they find their way? What do they do until they reach home? Until Menzel’s experiments, it had not been possible to track them in flight when they were out of sight out in the field. Menzel had a tool — radar but with a unique twist — whereby he could trace bees’ actual flight paths over a kilometer away by radar and record them on a computer. And he invited me to come see the work in progress.
Bee flight paths. A. A bee’s first trip from a flower patch or bee box back to its bee tree (hive) begins with an orientation flight. B. Later trips are more direct. C. After a bee has been transferred while “blindfolded” to a new spot, she acts as though she perceives herself to be still at the same place as before.
The problem of tracking small objects such as insects from a long distance by radar had always been that radar would “see” too much. You could not isolate and then plot a single specific bee out of all the extraneous noise of echoes bouncing off all objects. The new insect-tracking radar technique started in 1999, when Joe Riley, a British researcher, applied a radar system able to track very small objects over long distances by attaching to the insect a small device that, after receiving the energy of an electromagnetic sound pulse, would respond with a frequency other than that of the transmitted ultrasound. The receiver is then tuned to amplify only that frequency. In this way, it became possible to track the flight paths of individual preselected bees equipped with the appropriate transponders because the echoes from all other objects were filtered out.
The Menzel group’s electronics technician, Uwe Greggers, adopted the Riley system in 1999 and 2001 and got interesting results, but then ran into software problems. Nevertheless, given the promise from the data they did get, the scientists contacted a radar specialist at Emden (north Germany) who agreed to develop the system. The Menzel group then needed to find the right site in which to use it. They needed to locate the experiments at a large flat area devoid of trees in order to be able to record the complete flight paths without interference such as the bees’ trying to avoid objects or being attracted to them. The closest suitable area was an expanse of marshy meadow about a two-hour drive from Berlin. The large, idyllic farmstead near the village of Klein Lübben and land associated with it had accommodations for seven or more helpers, making this site amenable.
One Menzel group experiment in the works when I visited involved training individual bees to expect food at two widely separated feeding stations, but only one station at a time was open to them. I had no idea what to expect, and on my day with the team I was eager not only to watch the bees but also to see the experiment in action.
It was early in the morning when Menzel picked up Greggers and me for our trip to the experiment site in the Brandenburg countryside. We loaded a large, heavy printer that would be used to handle the large-scale printouts of flight paths, and then we were off down the Autobahn. Two hours later we arrived at Klein Lübben, a quiet village of farmsteads that at least in outward appearance has changed little since medieval times. The fields were several kilometers square, flat, and moist — perfect also for frogs, and hence storks which nest there in baskets attached to the tops of red-tiled house roofs. Swarms of starlings swirled through the air, and a pair of white swans paddled serenely down a canal along a dirt road, followed by a line of five still-downy gray cygnets.
At one end of the study field stood a steadily turning radar apparatus with a large round antenna for sending out the signal. A smaller dish antenna mounted directly above it would receive the transformed signal bouncing off the transponder on an airborne bee in the field. On the field sat two blue triangular tents and three yellow ones. They were experimental landmarks for bees that could be made available to them, to find out if they used them, and manipulated for experiments by changing their locations. In the distance sat a beehive, and I noticed a man running from it. He was wildly slapping himself, in an obviously defensive mode. He had been assigned to provide food for the bees close to the hive and then was to gradually move the feeder into the field so that a population of bees from that hive would be available for us to study when we arrived at midmorning. He had come too close to the hive, and at that moment it was he who was getting dispersed over the field, not the bees. Also, as we soon found out, there were no bees coming to the two feeder stations, as they were supposed to have been by now; the student had apparently overslept or been otherwise distracted from his assigned job of luring bees.
The experiment we wanted to do was in doubt. This was serious. Two hundred thousand Euros had already been spent on this study, and the boss was intolerant of negligence. Luckily, bees from hives used previously for another experiment were still coming into the field to search for feeders. He could let some of those bees find the feeders and then train them to come back to specific locations.
For our experiment we needed to establish two feeding stations, A and B, separated by about three hundred meters. Certain bees were already keyed into the routine. When I walked across the field, one bee started following me. It looked most extraordinary: it had a lot of blue and green color, not just the usual plain brown honeybee attire. As soon as Menzel’s helper and I set up our feeder, this specific bee landed on it and immediately started to suck up the rich sugar solution. Now I could examine her more closely: the green was a plastic tag with the number 29 on it that had been glued to her thorax. The blue was a slash of paint that had been daubed onto her abdomen.
Within a few minutes an assembly of several differently color-coded bees was lined up around the edge of the syrup dish. All were sucking up syrup. Some had green on the thorax, some had blue, and still others had yellow tags on their thoraxes, with additional daubs of white, blue, or yellow paint on their abdomens. Uwe Greggers and the unfortunate helper immediately started logging a list of the bees that had shown up in a notebook.
Each bee tanked up quickly, flew off directly toward her hive at the other end of the field, and then came right back to take a next load. Newly recruited (unmarked) individuals were also coming every minute to our site A. At the second feeder (site B) there was a similar flurry of activity, except it involved different individuals.
Menzel then instructed us to move our food station A one hundred meters closer to the second one, B. Bee numbers 29 and 30 green, both with blue tails, number 2 yellow with white tail, and number 39 green with red tail (who had all been present at A) then almost immediately started showing up at B, the new location. When crowds of bees had done the same, we removed one station and put the remaining one into the middle, between the two original sites. Next we moved our feeder to site B. Most of the bees, such as 30 green with blue and 39 green with red, who had been at the previous site, showed up. That is, we had trained bees who had been at one site to come to the second site, so we knew they now knew two sites and could potentially use either as a reference site to return home.
For the planned experiment it was important that the bees forget the intermediate sites that had been instrumental in getting them to go to the two widely separated sites. So, for the rest of the day, we alternately fed the bees, first at site A, then at site B, and monitored which individuals were showing up at both sites (most of the individuals continued to forage at either one or the other site).
We were now, near the end of the day, finally ready to move from training to trials. The experimental plan was to select one of the bees who knew both sites. This bee would, after feeding at one site and getting ready to leave, be captured in a dark box and thus “blindfolded” and then brought to a third feeding site where she had never been before. Here she would be released after being equipped with a radar-tracking transponder. We presumed that she would do at least one of three things: she might recognize where she was and fly straight home; she might instead fly off in her original (now wrong) direction; or she might immediately know that she was at an unknown location and search until she found one of her two feeding sites and from there take a direct beeline home. Knowing her exact flight path would allow us to distinguish among the alternatives, which would be essential to ultimately decoding her homing mechanism. Setting up this experiment had taken a long time, but I would now, possibly, be treated to an exciting demonstration of bee homing, one I could never have imagined possible.
Menzel picked up his walkie-talkie to call the radar station: “Mike, we’re now going to put a transponder on a bee — are you ready?” Mike had spent some years in the army where he was trained on radar, and he was now working part-time while getting a university degree in computer technology. He replied yes, he was ready. Menzel then took me to feeding station A, where a whole lineup of bees was coming and going.
“Which one do you want?” Menzel asked me. I wanted a bee that I had gotten to know over the course of the day, so I chose 39 green with red-tipped abdomen. We waited for her to arrive and let her feed for a while. As planned, Menzel then held a glass vial over her while she was distracted sucking up syrup. When she was full, she walked up into the vial, and Menzel corked it shut and darkened it by wrapping his hand around it. We then took her to a site distant from both feeders, a place where she had not previously fed and from where we would now release her.
The vial holding “39 green” had a plunger at the bottom with a wide-mesh screen at the top. Menzel gently pushed the plunger in and forced the bee up against the screen, held her there, and picked up a tiny transponder (a wire holding a diode with a sticky pad at one end). With fine tweezers, he deftly removed the protective paper from the sticky pad and glued the transponder onto the top of the bee’s thorax. “Ready?” he radioed Mike.
“OK.”
Menzel removed the plunger and held the vial with the open end up, for the bee to crawl up. She hesitated at the lip of the glass, groomed her antennae, and then lifted off. She showed no strain in flight. (The transponder’s weight is twenty milligrams, and a bee can fly with double her body weight, carrying a hundred-milligram load of nectar in her honey stomach plus two pollen packets on her hind legs.) However, she flew only two or three meters before dropping down into the grass, stopping to preen herself some more. But a couple of minutes later, she finally took off again. Mike, who was now monitoring her flight, radioed us. At intervals we heard: “She is heading south-north-east-north-west-south.” Then, finally, Mike continued: “Now her path is straightening out — now she is heading directly for her hive!”
She had suddenly oriented correctly. This was the crucial point: she had apparently recognized something that had “placed her on the map,” so that she then “knew” in what direction to fly to reach home. Assuming she had taken a path she had never taken before, did her successful homing suggest a “map sense”?
I ran over to the radar tent where Mike showed me the radar screen and the dots where the three-second successive readings traced the bee’s path. A computer screen, where software had converted the time and directions of the bee’s flight path into different-colored images for easy reading, showed that the bee’s original flight direction was toward where the hive would have been had we not moved her from her feeding spot. In other words, she acted as though she didn’t know where she was when we released her. As expected, however, after she reached the area where her hive would have been, she flew loops in several directions. Then, after she had flown ever-farther away from both her real and “would-have-been” hive locations, she suddenly seemed to orient and flew directly toward the hive. Amazingly, it was along a route that had not been her normal foraging route from her two feeding sites. Had she perhaps seen a blue or a yellow tent and, having learned their relationship to each other during previous orientation flights, transposed that information to fix her new location? Only more bees could tell.
Other bee homing experiments with hundreds of bees were ongoing. And in the group’s final publication two years later, the thirteen-author research team headed by Menzel concluded that honeybees incorporate information for flight direction from both their previously learned flights as well as landmarks and from the flight directions learned from hive mates within the hive. But they can redirect their flight vectors to and from the hive and perform novel shortcut flights between the learned and the communicated vectors.
“The” homing instinct, recognized and traded on by every American beeliner to get honey, and used by von Frisch to decipher the bee language, is a source of fascination and mystery still. Von Frisch had likened it to a “magic well” from which the more you take, the more runs back in. The “well” is still doing that, three-quarters of a century after his prophetic pronouncement.
GETTING TO A GOOD PLACE (#ulink_a1addad7-22a9-51d5-ad06-01f378599df8)
THE TENT CATERPILLAR MOTH, MALACOSOMA AMERICANUM, is common in North America. It emerges from its light yellow silk cocoon in late summer, and the female is then ready to deposit her batch of over a hundred eggs. She searches for an apple or a cherry tree, and somewhere out on a thin twig of just the right diameter — about a half centimeter — she exudes her eggs along with sticky foam to form her egg mass into a ring that wraps around the twig. The foam dries and hardens, encasing the clutch of eggs and gluing them solidly to the branch where they stay through the coming winter. But the larvae develop inside the eggs during the summer and, while confined in their eggs through the winter, hatch at almost precisely the day, about nine months after the egg-laying, when their tree breaks its buds.
The moth is named for the conspicuous communal homes of silk, called “tents,” that its caterpillars make, and in the spring of 2013 I found a just-made tent on a young black cherry tree next to my Maine cabin. Like nearly everyone else in this part of the country, I was long familiar with these caterpillars but had not deemed them worthy of a closer look. The tents act, I learned, like miniature greenhouses and warm the new caterpillars at a time when nightly frosts are still common. But, despite its advantages, to have any home is to incur costs: it has to be made, and it takes time, energy, and expertise to make, and the wherewithal to travel to and from it. For the time being, I wanted to know where the caterpillars making this home had come from. To my surprise, the ring on the twig with the now-emptied eggs I was looking for was almost a meter from the tent. How had the many hatchling caterpillars “decided” or been able to stay together and then coordinate to make their tent? Squinting against the sun, I could see a glistening trail of fine silk leading from the emptied egg-case ring to their home, so here was at least a hint as to how they crawl together to end up at the same place.
On the second day after I found the tent, May 1, there was still snow on the ground in the woods. There was as yet no sign of fresh green anywhere. But I wrote in my journal, “Black cherry buds ready to pop leaves.” These trees are the first to leaf out, and the caterpillars could not have fed yet. What would they do? An hour after the sun came up, the tiny caterpillars emerged from their tent and massed on its sunny side. An hour later they started milling about, and then a few started crawling, seemingly aimlessly, several centimeters up and down the trunk and branches of the cherry tree.
As I had anticipated, some of the tiny caterpillars started to crawl back onto the same branch they had come from, possibly following their previously made silk trail. But they went only six centimeters before turning back. Others went down the trunk of the tree. Always some would turn back, and then the others followed one behind the other in a line. Finally, by 7:30 a.m., a contingent of about twenty of them had progressed nine centimeters down the tree trunk, although two were coming back up. Then more started to leave the tent, and eventually all were in one long line, going only down the trunk and then angling up another branch. In half an hour the leaders had traveled seventy-three centimeters and reached a bud. The rest were strung out all the way to the tent, but their two other travel-direction options had been abandoned. All were eventually massed at the same cherry bud, three-quarters of a meter from their tent, and in an hour and a half they had all returned to their tent, one following the other in a long train.
The young black cherry tree showing relative locations of a tent caterpillar moth egg cluster (C) from which the clutch of just-hatched caterpillars emerged and traveled to start making their home (H) in a crotch of the tree, and their first travels as a group (T) to feeding places
At noon they came out and crawled onto the outside of their tent, waving their heads back and forth, apparently weaving silk from their salivary glands to enlarge it. Another hour later they were again all massed inside the tent and perched, immobile, tightly against the bark, where they were barely visible through the thin gossamer veil of silk.
The caterpillars stayed in their tent through the night, and I expected them to go at sunup to the same branch where they had been the day before. But instead, this time they all followed an entirely different path, going directly up the tree instead of down as on the previous day, and without taking another side branch. I could not detect any silk on their so-far two different foraging trails, and this time they went even farther — a distance of 130 centimeters. After their one meal the day before, they were already noticeably larger. A few were the same size as the day before, but most had probably doubled in weight. There were many tiny fecal droplets in their web. So they had fed, even though it seemed hardly possible that they had anything to feed on at the barely opening bud.
On the third day the buds had opened and the tree was replete with new small leaves pushing out of the buds. But it had been a cool night — there was again frost on the ground at dawn — and the caterpillars made a slow start.
The pattern soon became clear: the caterpillars spent most of the night and most of the day when they were not feeding in their home. The time spent on tree branches was brief, and it could not have been just to keep warm that they stayed in their home because they went back inside just as quickly after feeding regardless of temperature or time of day.
Having found and watched the caterpillars of one tent, I then observed others for more clues to their homing behavior. One of the surprises to me was that as they grew larger, they foraged independently of one another, no longer going to and from feeding areas in groups. Furthermore, after they were about half grown they left their tents, not to return at all but still to continue feeding before eventually searching for a spot in which to spin their flimsy cocoon. Tent caterpillars usually choose a bark crevice to pupate, although commonly they also choose the cracks in the sides of buildings. But why were the young caterpillars strongly homebound and the older ones not?
I suspect the young ones’ web-making behavior may have evolved in part as an anti-predator response. The tents were visited by red wood ants, Formica rufa, and right after the caterpillars hatched, these ants often loitered alongside them on their trails. I tore a nest open on one side to find out if it served as protection. It must have, because ants entered, though frequently wiping their antennae as though irritated. Nevertheless they tarried inside the damaged nest, and I saw them grab and walk off with caterpillars. No ants entered an intact nest of the several I watched, each of which consisted of several successive layers of silk. Thus, the webbing of the tent acts as a deterrent to predators such as ants. Staying inside the home most of the day and night, as these caterpillars appear to do when they are small, probably reduces mortality from parasitic flies and ichneumon wasps as well. When they are larger, the caterpillars are probably protected from the ants, as well as from most birds, by a layer of fine spines. They pupate without having to bury themselves to escape frost, because the adult emerges long before there is any frost.
Because these caterpillars are protected from predators in the summer homes they build and by the spines they wear, because they mature early enough in the summer for the pupae to avoid the cold of winter (by early emergence of the moth), and because the eggs and young larvae are immune to freezing because of the antifreeze they contain, “everything” in the life of the tent caterpillar moth may be found within a few meters. The adults that emerge in late June are not far from the apple or cherry trees where the parent left her eggs, and their life cycle can be completed without their having to go far from home, unlike some other insects which traverse a continent to be able to satisfy all their needs.
Monarchs. Of all the insects, the travels of the monarch butterfly, Danaus plexippus, are perhaps most famously spectacular in both scale and scope. Dr. Lincoln P. Brower of the University of Florida in Gainesville (now at Greenbriar College), who has studied this butterfly and its migration for over forty years, records the rich history of the emergence of our knowledge of monarch migrations. Early naturalists saw “immense swarms” in the prairie states where the caterpillars fed on the leaves of the many native species of milkweed (Asclepias) and the adults fed on the nectar of their flowers. Monarchs declined when later industrial agriculture destroyed many of their food plants, but in the nineteenth century they resurged in the East due to land clearing and the spread mainly of one milkweed, A. syriaca. Millions of them were seen passing for hours, even in Boston. This was a phenomenon that is hard to imagine now and it ignited much interest then. Charles Valentine Riley, the entomologist who first hypothesized that these butterflies engaged in a birdlike migration, cites people seeing them in the fall in swarms that extended for kilometers and obscured the sun, “blurring day into night.” Huge lines of them passing Boston in 1880 were described as “almost beyond belief.” Now, with reforestation, plowing, and then the use of Roundup and other weed killers that eliminated their food plants in agricultural fields, the monarch is but a shadow of what it was. In the past several years in the East, it seems to have almost disappeared. For the first time, I saw not a single one in late summer of 2013. But our knowledge of the scope of the monarch migration has blossomed.
Monarchs migrate on their own power for thousands of kilometers, and, unlike many other insect migrants, the population (though not the individuals) has a regular two-way migration, although as with the other insect migrants, the individuals that come back are not the same ones that left.
Unlike most of the other North American butterflies and moths, which overwinter in New England as eggs, larvae, pupae, or adults, monarchs cannot survive there through the winter in any stage. The population that normally now graces fields all along eastern North America overwinters at around three thousand meters’ elevation in dense fir groves on the southwest slopes of volcanic mountains thousands of kilometers to the southwest, near Mexico City. The monarchs find shelter in those fir stands from rain, hail, and occasional snow. It is not cold enough for the butterflies to freeze there, but it is cool enough for them to conserve the energy resources that they have accumulated on their way south.
The monarch butterfly adult, caterpillar, and chrysalis
In the summer, the monarchs fly in what look like random zigzag patterns over the New England fields as they stop here and there to sip nectar. Occasionally you see a mated pair, the female doing the work of flying, the male dangling passively with folded wings while attached by his genitals. After the prolonged mating (and/or technically “mate guarding,” since it prevents mating by other males), the female glues her delicately patterned green eggs with gold markings, one at a time, to the undersides of milkweed plants. In a few days, the flashy yellow-black-white larvae hatch and start chomping. After about fifteen days (depending on the temperature), the caterpillars have increased their weight to 1.5 grams (2,780 times the hatchling weight). The caterpillar attaches itself to a support such as the underside of a leaf by a clasping organ at the hind end of its abdomen to hang upside down. It will then molt into the bright green pupa (chrysalis) with the shiny golden spots that is surely familiar to almost all school kids. In a few days, the chrysalis starts to turn dark, and the outlines of the orange-patterned wings are visible through the now-transparent cuticle. When the chrysalis splits, along a predetermined line of weakness in the back, the limp adult slips out and expands its wings, and in two or three hours hormones will have instigated a biochemical process that hardens its body armor and stiffens its wings. The butterfly is ready to fly. Where will its wings take it?
Thanks to the monarch studies initiated in 1935 by Dr. Fred A. Urquhart and his wife, Norah Urquhart, from the Zoology Department of the University of Toronto and continued to the present day with the input and cooperation of thousands of amateur volunteers, there is now an amazing story to tell. The Urquharts noted in the late 1930s that the monarchs they saw in late May and early June in Canada had tattered wings, and they knew that this species would not and could not overwinter in Canada, so they suspected that they may have come a very long way. Monarchs fly in a southwesterly direction in the fall, but nobody had a clue where they ended up. To get some idea of the butterflies’ movements, these researchers in 1937 began gluing paper tags onto monarch wings with this message: “Please send to Zoology University Toronto Canada.” Monarchs weigh almost half a gram and the wing tags only 0.01 gram, so the tags were not likely to hamper the animals’ movements. Similar tags, used today, have pressure-adhesive backing and can be folded in half and glued over the leading edge of the forewing (after the scales are removed).
The idea from the inception of the monarch-marking studies was to try to find out if the butterflies migrated — an idea that at the time, as Urquhart noted, “was considered quite impossible.” But the question of where the butterflies might be going to and coming from grabbed the imagination, and anyone seeing a tagged butterfly would be sure to try to catch it. Sure enough, tags were returned over decades that suggested a migratory pattern. Individual tags were returned from huge distances, up to 1,288 kilometers. One monarch that was tagged in Ontario in 1957 was recovered eighteen days later in Atlanta, Georgia, 1,184 air kilometers distant. Clearly, when the butterflies left Canada in the fall, they headed south.
Still, nobody knew what happened to the mass of butterflies. Then, in January 1975, Cathy and Ken Brugger of Mexico City found them — a dazzling, shimmering, orange display of an estimated 22.5 million monarch butterflies on one 2.2-hectare site (which turned out to be only one of ultimately thirteen overwintering sites in the mountains of Mexico). The millions of monarchs were festooned in the trees in the mountains of Michoacán near Mexico City. The Urquharts excitedly traveled to see the site and on January 18, 1976, listened to “the sound of the fluttering of thousands of wings [that was] like that of a distant waterfall.” As they stood awestruck by this dazzling display, a pine branch broke off from the sheer weight of butterflies attached to it, and it crashed to the ground right in front of them. Fred Urquhart had been posing for a National Geographic photographer surrounded by these just-fallen butterflies when, incredibly, he saw a tagged one among them. When he traced its origin, he learned that it had been tagged on September 6, 1975, by Jim Gilbert, from Chaska, Minnesota. Urquhart, who had encountered countless tagged butterflies in his career, said it was “the most exciting one I have ever experienced.”
The picture that has now emerged from decades of study is that individual butterflies migrate all the way from Ontario to Mexico in the fall, arriving there at their overwintering sites in a torrent during October. They spend most of the winter in Mexico in a cooled low-energy state but soar around on warm days to drink water and replenish on nectar. In early spring, when their sex urge awakens, there is a mating orgy followed by a mass exodus. Most of the females mate before leaving, and their “compasses,” which were set to take them south in late fall, are now “reset” to take them in a northerly direction.
As the tide of butterflies advances northward, the females stop to lay their eggs on milkweed. Some of the butterflies from Mexico make it all the way to the north, and others (their offspring) that grow from the eggs laid along the way arrive later. Those of the first generation have slightly tattered wings when they arrive in the north, while those that arrive later have untattered wings. (However, not all monarch populations migrate, and not all that do, travel in the same directions as the populations of northeastern North America.)
One of the mysteries that puzzled Fred Urquhart was how the butterflies home. In Urquhart’s 1987 book on the monarch, he speculated that the butterflies perhaps use the Earth’s magnetic lines of force, although different populations of the butterfly migrate in different directions, so they could not all be orienting to it in the same way.
A potentially even more puzzling question is the ultimate (evolutionary) one of why these butterflies migrate in the first place. Urquhart simply suggested what he admitted was a “perhaps far-fetched” idea: that “twice each year it [Earth] passes through an area rich in some sort of radiation that could impinge upon animal life [that] might affect in some manner the cells of the body causing reproductive organs to abort in the fall and develop in the spring and initiate the migratory response.” This is an unlikely theory, though, mostly because it depends on a mechanism that is not adaptive in evolutionary terms. Instead, more current thinking about the adaptive reason why the phenomenon has evolved focuses on energy economy and maximization of resource use under the expected evolutionary constraints from the monarch’s having evolved in the tropics, meaning it was not able to survive northern winters. (Monarchs belong to the family Danaidae, an otherwise strictly tropical group.) Migration to the north in the spring opens up the milkweed crop over a major swath of North America as a food base for the larvae. In addition, the journey is probably not costly to the monarchs, either in terms of predation (since they are chemically protected from predation by poisons they sequester from their food plants) or in terms of energy costs, since their energy intake along the way more than makes up for the energy expended for travel. Indeed, unlike most birds that may deplete all their fat reserves on migration, these butterflies instead fatten up on their journey and may consist of about 50 percent body fat by the time they arrive in Mexico, where their overwintering fast begins.
Butterflies and moths experience tremendous selective pressure, and undoubtedly there are constant readjustments of survival strategies. Weather affects the populations, not only through flight activity and flight range as well as growth rates of larvae, but perhaps also indirectly by influencing virus infections. But Urquhart noted that each female monarch butterfly lays up to seven hundred eggs, and he calculated that the “biotic potential” — the number of individuals if there are no deaths — of one female after only four generations (that is, at the end of one summer) is 30,012,500,000 adults. Luckily for the planet, animals’ reproductive potentials are never naturally realized, for long. The limit is quickly reached when the population uses up its food base, in this case milkweed. In some years a virus decimates most of the monarch population over North America, but then several years later it rebounds. But the population cannot rebound from some things: in recent years there have been massive declines of the monarch population that cannot be reversed, because they are due to unnatural causes — the massive conversion of land to crops, and the introduction of genetically modified crops that tolerate herbicides, which have allowed the elimination of milkweed that formerly grew between rows of corn.
The flight performance of monarchs is spectacular, but like the hordes of cluster flies from the surrounding fields and woods that overwinter in my cabin, they are traveling to a specific place for overwintering where they have never been before. Such homing movements are diverse, but common. Robert D. Stevenson and William A. Haber of the University of Massachusetts, Boston, found a regular seasonal migration of about eighty percent (250 species) of butterflies living in the dry lowlands of the Pacific Slope of Costa Rica that migrate to wetter forests of the east. Distances traveled range from ten to a hundred kilometers.
In North America as well as in Europe, the cosmopolitan painted lady, Vanessa cardui, a mostly orange and black butterfly with white spots and pink and blue “eyes” on its under-wings, at times appears in large numbers and then is not seen again for years. Usually the individuals are seen crossing a road, and almost all will be heading in the same direction. The painted lady regularly migrates north from Mexico, from where it originates, after heavy rains in the deserts have created an abundance of food plants, primarily thistles. A friend told me of one migration while he was in Arizona when his windshield wipers “soon became useless” because of the huge numbers of painted ladies plastered onto them as he was driving. I see them regularly in Vermont and Maine, but seldom in large numbers (the summer of 2012 was one of the exceptions).
Red admiral butterfly larva, adult, and chrysalis. The larva makes a shelter for itself by pulling leaves together and holding them with silk, while then feeding on the leaf.
One of the butterflies that not only migrates as an adult but also hibernates in some parts of its range is the red admiral, Vanessa atalanta. It is (as are all butterflies!) beautifully colored. It sports a wide red stripe across each dark forewing ornamented with white spots, and its larvae feed on nettles. I wrote in my journal on May 11, 1985, near my home in Vermont: “In the afternoon from around 2:30 to 4:30 PM, as I was jogging along on an 18-mile circular loop I counted 512 red admiral, crossing the road in front of me. All but 5 of these were flying in a northeasterly direction. At 5:00 PM, after I was home, I take compass readings of butterflies flying over a plowed field where they funnel onto it through a valley. I can see them to take a bearing for at least 50 paces — 250 feet. All 22 that I observed flew in NE direction. At 6:00 PM activity almost stopped. The breeze is slight, from northwest.” In the summer of 2001 and again in the spring of 2010 I saw large numbers of red admirals. They fed on freshly opened apple blossoms, and later all the nettle plants in a neighbors’ sheep pasture had an abundance of their caterpillars.
Moth migrations are perhaps more spectacular than those of butterflies. Jason W. Chapman and colleagues report one recent ten-year study involving radar tracking of about one hundred thousand owlet (Noctuid) moths, primarily the silver Y moth, Autographa gamma, migrating south in the fall from northern Europe, and then north from the Mediterranean in the spring. Like the butterflies, these insects breed along their migration route. Also like the butterflies, the moths partially correct for crosswinds, to maintain specific directions. Most surprising perhaps is the moths’ windsurfing; they choose the most favorable wind currents corresponding to their respective spring or fall migratory directions. If the wind shifts about twenty degrees from the favorable direction, they adjust their flight to accommodate and maintain the correct direction. If the wind shifts ninety degrees, though, they stop and wait for a favorable wind. Millions of them fly together in the dark of night, and, like the monarchs’, their compass directions are likely tuned to the Earth’s magnetic fields. Some studies of radio-tagged green darner dragonflies, Anax junius, suggest that these insects also migrate hundreds to thousands of kilometers from north to south with those that return being a different generation.
These behaviors get the animals to a good place (for overwintering or for reproduction). Like the long-range movements with specific endpoints on the map, homing to a good place is not always easily distinguished from moving out of a bad place. The behavior is a mechanism with deep evolutionary roots. Indeed, insect wings (and metamorphosis) may themselves have been an original adaptation for dispersal, to colonize temporary pools, animal carcasses, or other temporary resources. The first individuals to reach the resource won the competition to use it and multiply there, and these were more likely to be the ones that flew, and flew far and wide, rather than those that walked at random.
Wings and metamorphosis have lesser value in constant conditions. Some insects are able to respond in real time to the changes in conditions they experience (especially crowding), in that when they don’t “need” to disperse they either don’t grow wings (some aphids) or the muscles to power the wings are broken down and the amino acids from the protein are used instead to make more eggs (in some Hemiptera bugs). Often there are discrete “dispersers” versus “non-dispersers” in any given insect population, and the percentage of each depends on the quality of the home habitat and hence the relative cost/benefit ratio of moving versus staying.
Dispersing to “anywhere but here” generally applies to nonmigratory species that have no encoded or learned directions to go to but may have innate instructions to move in more-or-less straight lines rather than potentially going in circles in order to achieve distance. In Africa, dung-ball-rolling scarab beetles race away from their often thousands of competitors at a dung pile at night by using the swath of stars of the Milky Way galaxy as a reference. Swarms of insects feeding at dung and carcasses also attract predators, and as soon as they finish feeding, many distance themselves from those predators. I’ve observed blowfly larvae at animal carcasses keeping to almost perfectly straight lines in their getaway at dawn, by steering directly toward the direction of the rising sun. Mass movements sometimes observed in some rodents, such as lemmings and gray squirrels (as in 1935 in New England) following a population explosion after a superabundance of food, may be another example of dispersal to get to a better place, though not necessarily a predetermined one.
On the other hand, “true” migrants are able to utilize ideal conditions in two places, provided they vary predictably. Arctic terns, Sterna paradisaea, breed throughout the Arctic, then fly to Antarctica to escape winter when food availability declines and to arrive in spring and food again, a round-trip distance of nearly seventy-one thousand kilometers. Gray whales, Eschrichtius robustus, also feed in the Arctic in the summer but then travel eight thousand kilometers along the coastline to Mexico to bear their calves in warm waters.
Dispersers are not neatly differentiated from migrants, although the first commonly rely on passive mechanisms as opposed to the migrants, which move to specific goals by their own powers of locomotion. There are all gradations in between. Each case is unique, and there are thousands. Let’s look at more ways of getting to a good place, as represented by eels, a grasshopper, aphids, and ladybird beetles.
Eels. There are many species of eels, but the American, Anguilla rostrata, and the European, A. anguilla, the species with which we are most familiar, live most of their lives in freshwater ponds and lakes. For reasons that are not clear, though, they do not reproduce in their home areas. To the contrary, both disperse (or “migrate”) thousands of kilometers on a one-way trip to spawn and then die in the mid-Atlantic.
Just as birds and some insects use air currents, eels use water currents to help them leave their lifetime homes. After eels leave their freshwater homes and head for the ocean to spawn and die, their larvae then drift in the ocean currents for years. But eels’ dispersal behavior is anything but passive. Eels fatten up to prepare to leave their home haunts in the bottoms of freshwater lakes and streams. Before they head for the sea, they absorb their digestive tracts and transform themselves by greatly enlarging their eyes and turning silvery on their ventral side. The latter transformation produces counter-shading that reduces their visibility to predators below them in the open ocean waters.
The eels’ dramatic changes in behavior, morphology, and physiology that enable them to switch from living in freshwater habitat to open ocean highlight the operation of strong selective pressures. But why do they leave their homes in freshwater ponds where they grew up and lived most of their lives? Their one-way, once-in-a-lifetime migrations to their Sargasso Sea spawning grounds can’t be to find a better feeding ground, or to escape competition. However, I suspect what the behavior accomplishes superbly is that the adults, which are predators, do not come in contact with and feed on their own young. Is it a mechanism that has evolved because it reduced predation on themselves?
There are over six thousand publications on eels, but the life cycle of these economically important food fish is still murky and has a long history of speculation. For centuries, nobody ever saw a baby eel, and even now, their spawning has not been witnessed. Aristotle presumed that eels grew from earthworms. The first young of eels, transparent leaflike forms, were found in the open Atlantic Ocean. Gradually, as more of these leaflike creatures were collected, it was noted that they varied in size, and that the smallest ones were found south of Bermuda in the Sargasso Sea, which was therefore presumed to be the site of their origin, that is, the eel spawning area.
The eel larvae, after hatching in the waters of the Gulf Stream, drift north. Like plankton, they move at the whim of the prevailing oceanic current. As they grow from about five to six centimeters in a year, they take on a more eel-like form but remain transparent. They are by then able to swim and, presumably by scent, find and swim up a river. Unlike salmon, however, these transparent “glass eels,” as they are known at this larval stage, can have no specific home stream scent to follow, because they have experienced only the scents of the ocean.
The female glass eels at this stage, in early spring, migrate up rivers and streams along the East Coast of America. After two months in a river they grow to about ten centimeters. Now known as “elvers,” they are no longer transparent, and they enter lakes and become eels. These female eels live in lakes for eight years or more, fattening up (the males stay in the saline estuaries). When they achieve the right amount of fat, these females become sexually mature. Each develops a clutch of three to six million eggs, and then one fall the gravid females start their journey downstream back to the ocean, to the Sargasso Sea to spawn. Since the males don’t live in fresh water, somewhere in the ocean the females then apparently meet the males for fertilization.
As the Gulf Stream continues north beyond North America, the larvae of the European eel, which originate from the same apparent spawning area, the Sargasso Sea, as the American eels, continue their journey. Finally, in two or three years, they reach the coasts of Europe, where they then also seek rivers and streams. Migrating upstream, they become pigmented, and after growing to adulthood, they migrate back into the open Atlantic and make their sixty-five-hundred-kilometer return to the Sargasso Sea, where they spawn on average several million eggs, and then die. Only one of several million of them will make the return journey to grow to a reproducing adult in fresh water.
Grasshoppers. One of the best-known insect dispersers, the “migratory locust” (the grasshopper, Schistocerca gregaria), engages in some of the most spectacular mass movements in the animal kingdom. On the African continent, this species has been famous since biblical times. Swarms of the “locust” have blackened the skies, and as those in the vanguard settle onto the earth and consume every green thing where they land, the rest fly over them until they reach more green, while those behind then take flight and do the same, and so a horde of hundreds of millions moves along, stripping all vegetation in its path. Predators cannot put a dent in those hordes. Additionally, the migratory locust is distasteful to potential predators because when it migrates, it is not choosy about what it ingests and takes up toxins from poisonous plants, which it incorporates into its tissues. The grasshoppers’ bright red-orange and yellow coloration, like that of many insects including the monarch butterfly, reminds potential predators of its distastefulness.
Nymphs and adults of the two phases of the migratory grasshopper
Although this distinctively colored grasshopper appears to arrive suddenly, it is often there all along, but in a different guise. It has a green solitary grass-fed form that blends in with its food and that is palatable to predators. For a long time scientists thought that the grasshopper “migrants” that appeared so suddenly were a unique species, one arriving from an unknown origin and heading for an unknown destination. Now we know that the migrants are a “phase” of a common species that changes its color, form, and behavior in response to crowding. Proof comes from experiments: to create these “migrants” from isolated individuals one takes a nymph (immature stage), puts it in a jar, and has a motor-driven brush tickle it continuously. The constant tickling mimics the crowding, which in the case of S. gregaria is the signal evolution has “chosen” to trigger the nervous system to alter the hormones that result in development into the restless migratory phase of a different color, wing length, and behavior. It is a good example that shows environment is “everything,” or from another perspective, it’s all about genetics.
Migratory-phase locusts are highly irritable and will jump up and follow a crowd flying over it. This behavior removes the grasshoppers from an area that is overpopulated and brings them to new land where conditions are conducive to feeding, egg-laying, and growth of their offspring. Although the grasshoppers could have no knowledge of where such a distant but good place might be, they migrate to it as if they do.
The grasshoppers reach a consensus. It is a sensible one, although it involves no thinking and no discussion. They simply fly up to join the crowd, which follows the prevailing winds. Eventually these winds meet air from an opposite direction and, when moist tropical air rises into cooler altitudes, rain precipitates out of the resulting clouds, depositing the grasshoppers to earth along with the rain. As the ground is watered and softened, the grasshoppers can shove their abdomens into the soil to lay their eggs. The new nymphs hatch just as new food starts to sprout. Their homing (or “dispersal”?), which has ended at this good place for them to reproduce, is now complete.
Aphids live in crowded “colonies” on plants into which they insert their mouthparts, much as mosquitoes puncture skin, except that they imbibe plant sap instead of blood and may stay plugged in at the same spot for most of their life spans. One might suppose they could not or would not migrate. But, like the migratory grasshoppers, they may travel possibly hundreds of kilometers. Nobody knows for sure how far; it depends on the prevailing winds.
Sedentary aphids already on good food do not leave to seek, or even require, mates. Instead, they switch to virgin births after a sexual migratory phase. Daughters then settle directly next to mother, and so on and on for many generations as the colony grows. And then, cued by the shortening of the days in the late summer and fall when the food supply runs out, the aphids’ offspring take a different developmental route. Because of changing day length and/or food, the nymphs on their final molt grow wings and become sexual. Frail and weak-winged they are, but an aphid is light and carried by the wind much like the seed of a dandelion or poplar tree, or a baby spider on a thread of silk. I usually see them in September when they appear like flecks of white lint floating erratically in the air.
To reach wind the aphids fly or are wafted up. Eventually, they don’t fight the wind but drift along and settle somewhere back down to earth. On their descent, assisted by their wings, they head toward anything colored light green. This color (unless they are tricked by pieces of green paper coated with sticky glue left by an insect physiologist studying them) is likely to be associated with their favorite food, fresh plant growth. After landing, perhaps because the chances of a mate arriving at precisely this one tiny spot of residence are remote, they switch back to virgin births and thus restart the cycle.
Ladybird beetles, the predators of aphids, similarly have adapted by migrating in a seasonal environment. In the western United States they migrate mainly on their own power from lowlands up into the Sierras, where they can in some locations be scooped up by the bucketful (generally to be sold to farmers and gardeners — to control aphids!). At the campus of the University of California at Berkeley, I often saw streams of them flying or being blown uphill in Strawberry Canyon by the campus when the grass was drying after the spring rains.
Ladybird beetles of some species migrate when reproduction must cease for the season. Despite the energy they expend for flight, they may migrate largely to save energy. It goes this way: As long as there are plenty of aphids to be had, both larvae and adult ladybirds don’t go hungry. Eventually, however, the green vegetation suitable for aphids disappears in the hot California summer, and so the aphids leave. Now the beetles’ resting metabolism kicks in as a significant liability. Resting metabolism for beetles is high at high body temperature but becomes almost negligible when they are torpid at the lowest body temperature tolerated, near or slightly below freezing. An elevated resting metabolism, month after month in the western states’ dry hot summer, would deplete both the beetles’ energy reserves and their body water. Without replenishment of food and water in that environment they would die. But by flying, with the aid of thermals, they are brought into upwelling air currents in the hills and then into the mountains where they reach cooler air. At this point, though, they do something different from the aphids: instead of being attracted to green, they are attracted to either red and/or the scent of each other. How else to explain that they crowd together into large groups in which they then overwinter? The advantage of their grouping behavior is not clear, but I suspect that it amplifies their noxiousness. (This is based on experience: ladybird beetles regularly come into my cabin to overwinter and quite often crawl into bed with me. I can vouch for the fact that they are noxious if not obnoxious, and several more so than one.)
The ladybird beetles arrive at a suitable place — a cool one — where they conserve their limited energy reserves during winter. The hypothesis that they home not just to an area, but also to a specific spot, is based on observations that my friend and colleague Dr. Timothy Otter has made in the Sawtooth Mountains near Stanley in Idaho. The beetles there were known by local ranchers to aggregate every fall in large numbers in specific rock cairns of decomposing granite in the hills above the valley floor. Otter, a biologist, wondered why the beetles aggregated there, in specific spots, and not in similar places nearby.
Since hibernation concerns adaptations related to temperature, Otter concentrated his efforts on unraveling the beetles’ temperature tolerance and compared it to the temperature at that site. But, curiously, there was nothing unique about the temperature of the specific site on “Ladybug Hill,” relative to other sites near it. Nor did the grouping of the beetles affect their temperature; the temperature of their aggregation was indistinguishable from ambient temperature. So, they didn’t aggregate to keep warmer than the environment there.
As already mentioned, the one thing all ladybug beetles do have in common is that they stink. The evolutionary significance of their aggregations is therefore likely the conventional one of other animals, namely, to advertise their noxiousness for protection from predators. Shrews and other predators may kill two or three victims and then spit them out, but they then learn to avoid them and so they do not continue eating the hundreds of thousands they would consume if they had found a bonanza of ladybirds. Given their safety of numbers, it is advantageous for any one beetle to join a group rather than to overwinter alone, because the chances that it will become a victim of a shrew’s educational process are reduced in more or less direct proportion to the group number. This rationale for large numbers of beetles, but of different individuals, massing repeatedly at the same location is not proven, but my colleague Daniel F. Vogt and I found that it applied to another noxious-smelling beetle, the whirligig (Gyrinidae) water beetles in Lake Itasca in Minnesota. These beetles there homed in on any existing groups of tens of thousands at dawn, after a night of foraging far and wide on the surface of the lake.
The second question is: How is an aggregation formed?
Insects have an impressive ability to home in on scent, and ladybird beetles could find the aggregation by following an odor plume. Memory cannot be involved in the ladybugs that Otter found year after year aggregating at the same site, which were generations removed from those of the previous year. The beetles arrive on site in September, stay there eight months, and in May return to the valley floor to feed, mate, and reproduce. Only their descendants, two or three generations later, could return to the tiny spot where they had hibernated.
The aphids’ rule of flying up toward the light, to then be dispersed by the wind, and then homing in on green when they settle could be a model of what happens in ladybird aggregation. Ladybirds are much stronger fliers than aphids, although they too are swept along in updrafts. But such drifting, while helping to account for their annual ascent from the lowlands into the mountains, does nothing to explain how tens of thousands of them end up under the same rock pile.
If the ladybirds home in on color, this could be tested, as the aphids’ homing in on food plants was tested — by leaving color targets at sites other than the traditional hibernaculum. But even if red color is an attractant (highly unlikely because the beetles aggregate under the rocks, not on them), that still would not explain their annual return to the same place in successive years. Do they smell their ancestors? Could thousands of smelly beetles piled up for eight months leave sufficient scent residue to serve as a marker that allows others to home in on the spot? If so, this idea could also be tested, by transferring an aggregation of beetles to overwinter at another physically similar place in the same general area as the old to see if a new traditional homing site for their mutual protection is created.
BY THE SUN, STARS, AND MAGNETIC COMPASS (#ulink_476b060f-b928-5d0d-9e35-7a5aea75144f)
Life has unfathomable secrets. Human knowledge will be erased from the world’s archives before we possess the last word that a gnat has to say to us.
— Jean-Henri Fabre
CHARLES DARWIN REFERRED TO THE ACCOUNT OF FERDINAND von Wrangel’s Arctic explorations, The Expedition to North Siberia, concerning how we home, quoting von Wrangel on how the Siberians oriented by “a sort of ‘dead reckoning’ which is chiefly affected by eyesight, but partly, perhaps, by the sense of muscular movement, in the same manner that a man with his eyes closed can proceed (and some men much better than others) for a short distance in a nearly straight line, or turn at right angles and back again.” Darwin compared a bird’s homing capability with that of people, but much less favorably, by telling how John James Audubon kept a wild pinioned goose in confinement, which at migration time became “extremely restless, like all other migratory birds under such circumstances; and at last it escaped. The poor creature immediately began its long journey on foot, but its sense of direction seemed to have been perverted, for instead of traveling due southward, it proceeded in exactly the wrong direction, due northward.” I’m not the least surprised at the behavior of the goose but all the more puzzled by our own orienting, which involves knowledge versus a feeling of “sense of direction.” I recall instances of waking up in “total” dark, “knowing” in my mind precisely how I am oriented relative to the room and hence the rest of my environment, but irked by “feeling” that I am in the precise opposite direction. It is then a struggle to get the two to agree, which happens only after some effort.
In Darwin’s time it was still supposed that humans had overall superiority over other animals. His then-hypothesis (later theory, and now fact) of evolution, which now binds us all as kin, was still revolutionary. Darwin found the goose’s behavior puzzling because he could not know that geese, cranes, and swans stay together in family and larger groups and that although the young by themselves do not know the correct migration route, they learn to know it from their parents which in turn learned it from theirs. Other birds have their migratory directions genetically coded, and they go strictly by “feeling,” since many of these have no knowledge because they migrate ahead of their parents.
We humans get lost easily. We would not get far without reference to landmarks, and I base that conjecture on (inadvertent) experiments. In one I was in long-familiar woods and got caught in a heavy snowstorm. Suddenly I got “turned around,” and it seemed as if all landmarks had in almost one instant been erased. But I kept going, trying to maintain a straight line by trusting my “sense of direction.” When I thought I had reached a place that I knew, where I should be going downhill, the landscape was instead sloping upward, and the brook I had expected to see was going in the “wrong” direction. At that point, knowing I was lost and no longer referencing to any signal, I backtracked in the snow and discovered that I had been walking in a circle, all the while thinking I was heading in the “right” direction. Yes, we can walk, a short way, in a relatively straight line with our eyes closed, by a process dubbed “path integration,” but my emphasis here is on “relatively.” Mice may do better. A friend told me of catching a Mexican jumping mouse in a live trap baited with peanuts. It had a kink in its tail, so he called it Crooked Tail, or CT for short. After he had released it several times, and it always returned for another snack of peanuts at the same source, he finally decided to “test its mettle” and released it exactly one kilometer away in thick brush and grass. The next morning CT was back for another snack. After release from two kilometers, though, it did not return. We don’t know, though, if this was due to failed navigation, finding a new food, a cost/benefit calculation, or a run-in with a coyote, owl, or weasel. On the other hand, when I failed to navigate, I was positive that I had been going in the precise opposite direction, which meant I had no sense of direction whatsoever, except that coming from visual landmarks from which I had constructed a map in my head.
When we do home, it is by maintaining a constantly updated calculation from at least two reference points, and the motivation to use them. We are innate homebodies, normally seldom displaced, so that in our evolutionary history there has been little need for a highly developed home-orienting mechanism. Simply paying attention to familiar landmarks suffices. Males on average may perform better than females in negotiating unknown territory, and it is posited that they, having been hunters traditionally, have a better “sense of direction.” But I doubt it. Learning, and especially attention, is hugely important for a presumed directional sense that can be developed to a high degree, as shown in some Polynesian seafarers living on isolated islands. But basically that involves being alert to more cues. These seafarers had been trained from near infancy to “read” the stars, the ocean waves, the winds, and other signs so that they may navigate over vast stretches of open ocean. But what a select few human navigators can accomplish with experience and with tools, many insects and birds do routinely as a matter of course, and with far greater precision over distances that span the globe.
Every fall and spring billions of birds travel to their wintering grounds where they can find food, and in the spring they return to near where they were born in order to nest. In huge tides, partially aided by favorable winds but mostly by their own muscle power, they ply the skies in the day and at night in the Northern and Southern hemispheres, sometimes covering thousands of kilometers in a few days. For the most part, the birds have pinpoint home destinations, places such as a specific woodlot, field, or hedge. In the fall they reverse their journey, though often by a different route, again to reach specific pinpoints in their winter homes. Turtles on the seas accomplish the same navigation feats between breeding and feeding areas.
The magnitude of birds’ migratory performances staggers our imagination, in terms of both physical exertion and feats of navigation, because they are vastly superior to anything we could, as individuals, accomplish. Bird migration, as we now understand it, for centuries seemed impossible because we used ourselves as the standard, and that of turtles was not even considered. The animals’ performances would still seem impossible, given our ignorance and arrogance, were it not for the proof from countless research experiments.
The homing behavior of birds was known and used as early as 218 BC, when Roman foot soldiers captured swallows nesting at military headquarters and took them with them on their campaigns. They put threads on the swallows’ legs with various numbers of knots to specify perhaps some prearranged signal or information, so that the marked bird when released and then recaptured at its home nest would bring the message. Today, between 1.1 and 1.2 million birds are banded annually in America alone, providing an ever more detailed picture of where the different species travel and when.
As with insect dispersals/migrations, our attention and insights into bird homing were and still are stimulated by spectacular examples. We are perhaps most impressed, if not baffled, not only by the birds’ wondrous physical capacities, but also by the cognitive or mental capacities that underlie them. Seafaring animals, like albatrosses and shearwaters and sea turtles, are especially noteworthy to us because we can’t explain their behavior by the use of at least to us visible landmarks, our main if not only recourse.
The Manx shearwater, Puffinus puffinus, navigating over the vast oceans, was one of the first birds to excite our curiosity enough to spark examining the wonder of bird homing. Shearwaters never cross land. All their food is taken from the water surface. As with most birds, their young are fixed to a specific safe or sheltered place, in this case an island, where one parent may spend as much as twelve days at a time ceaselessly incubating before being relieved by its mate. They nest in a burrow in the ground on islands in the North Atlantic, making it quite easy to catch, mark, and release them to identify individuals. We can also assume that as with bees, their motivation is to return home, and thus they are ideal subjects for homing experiments.
Prior to the First World War, the English ornithologists G.V.T. Matthews and R. M. Lockley took two shearwaters from their nest burrows on the island of Skokholm off the southwest coast of Wales and released them from points unknown to the birds. Under sunny conditions, the shearwaters returned to their nests by flying directly in their homeward direction. In one such test, a shearwater was carried by aircraft to Venice — a huge distance from its nest and an area where no shearwaters occur. The released sea bird might have been expected to fly south to the sea. Instead, it headed directly northwest to the Italian Alps and in the home direction toward Wales, in a path it never would have flown before. It returned to its home burrow on Skokholm 341 hours and 10 minutes later. This could, of course, not have been a direct nonstop flight. Unfortunately at that time there was no way of knowing if it had stopped to forage and/or what route it had taken.
The experiment was repeated involving even greater distances, after transatlantic plane travel became routine. Two banded Manx shearwaters also taken from Skokholm were carried by train to London in a closed box and flown to Boston, Massachusetts, on a commercial TWA flight. This is perhaps the ultimate in terms of the “blindfolded” displacement that I previously described for experiments with honeybees. One of the birds did not survive the journey to America, but the other, which was released near a pier on Boston Harbor, “abruptly turned eastward over the ocean.” Dr. Matthews, a leader in the study of bird homing at the time who had released 338 Manx shearwaters on the British mainland, discovered the bird back in its home burrow before dawn on June 16, twelve days and twelve hours after it had left Boston, almost five thousand kilometers away. On reading its tag, Matthews sent a telegram to the person who had released the bird: “No. Ax6587 back 0130 BST 16th stop-FANTASTIC-MATTHEWS.” Making another round that night to check on the bird again, Matthews, as though not believing his eyes the first time, wrote in a letter (to a friend, Rosario Mazzeo) that he was “completely flabbergasted” and had to read the ring several times before putting the bird back into its burrow.
By 1994 biologists had attached radio transmitters to animals that sent out high-frequency radio pulses received by satellites orbiting up to four thousand kilometers away. When two satellites picked up the same signal, scientists could calculate the transmitter location and relay it to receiving/interpreting sites on the ground. There, computers tracked the birds’ positions and drew maps of their travel routes over months. From these and other studies, we have learned that these seafarers, and sea crossers, both turtles and birds, may wander over thousands of kilometers of the ocean vastness and then return to tiny isolated targets, the homes where they were born. They can travel in straight lines even at night and while correcting for the drift of currents or wind. Using the new technology, these behaviors have been demonstrated perhaps surprisingly in a sandpiper, the bar-tailed godwit, Limosa lapponica baueri.
The bar-tailed godwit, a shorebird that nests on the Arctic tundra, winters in the far south of Australia. It has a long thin bill for extracting worms from deep soft mud. This species makes its Arctic home on a shrubby hillside with low tundra vegetation and nests there on almost any of millions of hummocks to be found on the tundra in Alaska or Siberia. Its nest is a slight depression lined with grass and lichens. The female lays her clutch of four large olive-brown mottled eggs into it, and the pair take turns incubating for about a month until the fluffy young, in camouflage down, are hatched. The parents then lead their chicks around and they feed themselves.
The bar-tailed godwit is not a particularly unique shorebird, as such. (The Hudsonian godwit, Limosa haemastica, performs similar flights from Manitoba to Tierra del Fuego and back.) But in the past ten years, possible extremes of homing ability and some astounding physical capacities that back up this behavior have been revealed by Robert Gill Jr., a biologist with the U.S. Geological Survey, who deployed twenty-three godwits with either solar-powered backpack transmitters or battery-powered surgically implanted ones in the abdominal cavity. The transmitters trailed thin antennas behind the birds, and the radio signals from them indicated their location and were received by polar-orbiting satellites. The data of the godwits’ locations throughout their flights was then calculated on the ground. Nine of the transmitters functioned for two years, yielding data on both the southern fall migration to Australia as well as the spring migration back home to the breeding grounds in Alaska.
Flock of bar-tailed godwits on migration
They revealed the hugely surprising fact that the godwits make the flight from Alaska to Australia nonstop.
The godwits fly directly across the Pacific Ocean in six to nine days. One female covered 11,680 kilometers in 8.1 days in her southward migration, and another traveled 9,621 kilometers before she lost her transmitter after 6.5 days. When the birds arrive back in New Zealand or Australia after their transoceanic flight — with no feeding, no drinking, and presumably no sleep — they have halved their starting body weight.
Portrait of a bar-tailed godwit
The godwits’ northward journey to the breeding grounds may involve a different route, and this one includes stopovers on the way. These stopovers permit the birds to replenish so they don’t arrive emaciated just when they begin the most energy-demanding part of their breeding cycle. For example, one godwit, identified as “E7” (which covered twenty-nine thousand kilometers in a round trip from New Zealand to its nesting area in Alaska), on its northward journey stopped at several staging (refueling) sites in the western Pacific and Japan, from where it then made the relatively short jump to its western Alaska home. On the other hand, on its southern migration after the nesting, it flew directly south from Alaska across the Pacific and back to New Zealand.
Right after a male godwit arrives back at its patch of tundra that is its home in Alaska, he circles for hours high in the sky and calls loudly near this chosen home site. In as little as a week before, he may have been on a coastal mudflat in Japan, where he had a raging appetite and gobbled worms and crabs day and night. Similarly, to prepare for his departure before the Alaska winter freeze-up in the fall, he will feed until he has doubled and even almost tripled his body weight in fat. And then, by our standards, in grossly obese condition, he lifts off to fly south. Although some godwits will stop off briefly in the Solomon Islands and New Guinea, others will fly up to fifteen hundred kilometers per day without a single stop. On their stupendous flight the godwits use up not only their body fat but also protein derived from shrinking muscles and organs, including almost every part of the body except the brain. The flight muscles are the primary powerhouse for the effort, but the brain — the organ that drives birds’ motivation to keep going — is more important.
Why do the birds leave at all, or go so far? Why do they face the privations, risks, and exertion of the journey? What drives their rapid fattening up without which they could not have enough fuel to reach their distant destination? Only raging appetite would fuel the fattening. Only an unquenchable drive to fly would make them go and keep going. The motivations and the behaviors presumably evolved because the Arctic summer provides more food than farther south, and so many species became adapted to be at home in that habitat. On the other hand, the Arctic provides little sustenance for most in the winter. The great migrations were shaped, then, by these imperatives.
I may be anthropomorphizing to suggest the godwits have a “love” of home, but although we can never know what they feel, it is hard to deny that they do feel. We can say that, along with the aforementioned cranes Millie and Roy, it is highly unlikely that conscious logic could drive them from one continent to the next. Animal behavior is first of all driven by emotion, although in us the emotion can be secondarily buttressed and/or amplified by logic. That said, we admire emotions that help us accomplish great things. We admire the drive and commitment that the birds show because our individual extraordinary feats pale in comparison to those of a godwit. The first lizards that sprouted feathers on their forelimbs could shield themselves from the rain and cold and may have been able to glide several meters, but for that they probably did not need drive related to homing. To fly nonstop for eleven thousand kilometers over open ocean, though, without taking a bite of food, a swallow of water, or a minute of sleep, is a mind-boggling demonstration of the epic importance of home, and of the ability and drive to return to it of even tiny birds.
Consider the example of a common European garden warbler, Sylvia borin. It is born in May somewhere on the northern European continent. It never in its life receives any instruction on when and where to fly. But two to three months after its birth it begins its flight in the night to Africa, where it has never been before. After reaching the Middle East, having flown in a generally southeastern direction, it shifts into a direct southerly direction and crosses the Sahara Desert. It eventually ends up somewhere in a patch of thorn scrub in perhaps Kenya or Tanzania, where it remains until spring. It then returns not just to the north, but perhaps to the same hedge in Russia or Germany from where it came, and after nesting there it again flies south to Africa to the same patch of thorn scrub where it wintered before.
Songbirds in North America do much the same. The Bicknell thrush, Catharus bicknelli, lives in the summer in the spruce forests on mountains not only directly adjacent to my home, but throughout the mountains of New England, the Catskills, and eastern Canada. It spends winters in the cloud and rainforests of Jamaica, Cuba, Dominican Republic, Haiti, and Puerto Rico. Christopher Rimmer and Kent McFarland and colleagues have been tracking these endangered birds in both habitats, to determine their home requirements. McFarland is the associate director of the Vermont Center for Ecostudies and has banded nestling Bicknell thrushes in Vermont. The birds return annually to their same homes, and his first encounter of an overwintering thrush in the Dominican Republic turned out to be one he had banded nineteen months earlier in Vermont. He told me that capturing the same bird seemed like “winning the lottery while at the same time being struck by lightning. But for us naturalist types, much more exciting.” On this occasion he broke out the celebratory Dominican rum on the very first night of that trip rather than toward the end of the fieldwork, as is more typical.
Routes of long-distance homing are now well known, but the how of the travel and the orientations to specific points of destination are still tantalizingly far on the horizon. The how is the most challenging of all to comprehend fully, because it literally involves everything about the animal at once — senses, metabolism, emotions, mechanics — all the physiology that runs the brain and the rest of the body. Solving such problems requires access and repeatability; animals don’t migrate in the lab at one’s convenience. Only one piece, or a few interrelated pieces, of the puzzle can be profitably examined at a time. Usually one animal species, by some quirk of its biology, provides access to a specific piece of information and another provides an opportunity for access to another.
The common rock dove or “pigeon,” Columba livia, with its long association with humans, has provided clues to many aspects of homing. The same or similar general homing mechanisms of this “homing pigeon” could presumably also be used by migrant birds, and nonmigrating but far-ranging sea birds and turtles. Pigeons were well known since the Assyrians and Genghis Khan, who used them in war. Julius Caesar used them to send messages home from Gaul. They were used in the two World Wars and in the Korean War. Because of their attachment to home, they were ideally suited, as were swallows, for carrying messages, especially in wartime, as they were difficult to intercept and were probably more reliable for transmitting secret messages than the telephone and Internet are today. Fifteen-hundred-kilometer flights for birds in the U.S. Signal Corps are considered routine, and flights of twice that distance are recorded. One could release pigeons at any location and at any time and be assured they would try to return home, provided they were not too young.
One of the common sights wherever pigeons are kept is groups of them circling near their lofts in apparently aimless flight. Pigeons engaging in these flights are said to be “ranging” — they may be out of sight of the home area for a half-hour to an hour and a half. As in honeybees starting their foraging career, these flights are especially important for the young birds because during them they familiarize themselves with their home area.
Are the pigeons, like bees, using landmarks for homing? To test for this possibility, Klaus Schmidt-Koenig and Charles Walcott, both renowned bird orientation experts, put frosted contact lenses on pigeons’ eyes to prevent them from seeing landmarks. To everyone’s surprise, some of the pigeons, after being displaced, still managed to return to their home lofts. They flew in at high elevation and then fluttered down close to their home. The birds had apparently gathered some clues other than landmarks visible to us.
Through time and experience, and longer and longer ranging excursions, pigeons enlarge the area where they are at home. A working hypothesis is that “lazy” fliers, those that make only short flights, are unlikely to be able to home from long distances. Pigeon racers, who compete in the homing ability of their birds, bank on the knowledge that the longer the ranging flights, the swifter and the more accurate the homing ability. After about two weeks of ranging, the pigeon racer usually takes his or her pigeons farther away for each “training toss.” Typically, the first training tosses are about thirty kilometers from the home loft. After three weeks the distance is increased to sixty kilometers, and then after another week to ninety kilometers. The birds’ capacity gradually to increase their homing ability reinforces the notion that they are learning something about their home area, perhaps something like a “map” using some kind of landmark. Precisely what the birds are sensing at any one time that allows them to orient correctly to return home is not known, in part because it probably varies depending on the place and the situation. Although it is still not clear exactly how pigeons are able to home, we know that several senses are involved.
We have seen that some migrant birds stay together in family groups (geese, swans, and cranes), and that the migratory directions are learned from the parents, which expose the young to the relevant cues much like pigeon fanciers expose their charges with “training tosses” far from the home loft. The phenomenon of parental leading has been documented in whooping cranes, Canada geese, and ibises and extended by humans leading young tame birds to become imprinted on ultralight aircraft, in order to establish new migration routes. In most migrant birds, though, the migratory directions are inscribed in a genetically fixed “program.” In either case, the migrants travel between one fixed territory in their summer home, and another in their winter home. However, presumably other, especially complicated mechanisms of homing are required in sea birds, which range far over the oceans and sometimes return to only a tiny speck, their natal island, after having wandered from it five or six years before. Do they build a map in their brain of some features of the ocean terrain that we can’t see? In other words, do they see the ocean not as a flat, uniform expanse as we do, but instead as a featured pattern as of hills, valleys, ridges, and mountains in perhaps magnetic anomalies that inform them where they are at all times?
The one thing we now know for sure is that, like us and like bees, birds use the sun as a compass for homing. Gustav Kramer, a German ornithologist, perhaps the principal pioneer in homing behavior in birds, in the late 1940s tested the “sun compass” of pigeons in circular cages with food cups placed regularly around the periphery. The birds were trained to expect food in specific cups (directions). After the pigeons were trained, rotating the cage did not alter the direction where they sought food — except when the sky was overcast and the sun not visible, when they searched randomly for food at the different cups. Kramer repeated similar experiments with a well-known bird, the northern European starling, Sturnus vulgaris.
European starlings in Europe migrate south in the fall (though many or most of those now in Vermont and Maine do not), at which time they, as well as other migrants, enter a state of restlessness. Kramer coined the word Zugunruhe, meaning literally “migratory restlessness,” to describe it. He first noted this behavior in his caged starlings, which were agitated and hopping around in their cages in the spring and tended to orient northeast. They oriented in the correct migratory direction when the sun was out, but as soon as the sky was clouded they no longer oriented in any one direction. Suspecting that, like the pigeons, they might use the sun to orient by, he tested his hypothesis by showing them the sun in a mirror and found that they then reoriented to the reflected sun. But the sun moves through an arc from east to west throughout the day, so how can the birds keep a constant migratory direction? Was the starlings’ behavior a laboratory artifact?
In order to find out if starlings indeed adjust the angle of flight to the sun throughout the day, Kramer put his migratory restless birds into a room where they did not have access to sunlight. Instead, he provided a stationary light bulb to stand in for the sun. As predicted, if they used the light bulb as a substitute for the sun and possessed a time-compensated sun compass, the birds oriented increasingly to the left throughout the day. That is, they changed their intended flight direction with respect to the constant light bulb direction, treating it as though it were moving on the same schedule, of fifteen degrees per hour, as the sun does, and so they almost always faced in the “wrong” migratory direction in reference to the ground.
Kramer later lost his life while climbing a cliff trying to get baby pigeons to raise them for further experiments on homing orientation. But one of his students, Klaus Hoffmann, carried on his work. Hoffmann, who later worked at the Max Planck Institute for Behavioral Physiology in Germany, nailed the “time-compensated sun compass hypothesis” with another experiment in which he “tricked” starlings to misread the time from the sun’s actual position. Given that the sun changes position fifteen degrees per hour, to keep flying in a straight line using the sun as a landmark, the bird has to know what time it is in order to compensate for the sun’s shifting position. Hoffmann kept starlings in an artificially lit cage with a normal twelve-hour period of daylight, but with the lights coming on six hours earlier than actual dawn in the real (outdoor) day. These birds adjusted their activities to the artificial light schedule they experienced and expected food at a specific time in one specific direction in a circular cage, and their feeding time was of course six hours ahead of real or solar time. When his “clock-shifted” starlings were trained to expect food at their food cup in a specific direction and tested under a stationary light, they oriented ninety degrees (or fifteen degrees for every one-hour time shift) in a clockwise direction from their training dish. This experiment confirmed, by a different experimental protocol from Kramer’s, the astounding hypothesis that the birds not only use the sun as a directional compass but, like bees, also consult an internal clock to correctly compensate for its rate of movement through the sky. Clock-shifted monarch butterflies also orient in the “wrong” but predicted direction, showing that they also use the sun as a “landmark” in migration.
This sophisticated behavior of insects and birds, however, does not explain the majority of homing orientation. Most songbirds migrate mostly at night, when they could not have access to the sun’s location as a convenient directional beacon. (It is likely that small songbirds have to migrate at night because they need the daytime to replenish their energy supplies by feeding, whereas large birds, like huge airliners, have a longer flight range and burn much less fuel in relation to their body weight.) For a long time it was not known how, with neither landmarks nor sun available, the night migrants might orient. Yet orient they did, as experiments on warblers (Sylviidae) by Franz and Eleanor Sauer proved in the late 1950s.
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Bernd Heinrich
The story and science of how animals find their way home.Home is the place we long for most, when we feel we have travelled too far, for too long. Since boyhood, acclaimed scientist and author Bernd Heinrich has returned every year to a beloved patch of woods in his native western Maine. But while it’s the pull of nostalgia that informs our desire to go back, what is it that drives the homing instinct in animals?Heinrich explores the fascinating science behind the mysteries of animal migration: how geese imprint true visual landscape memory over impossible distances; how the subtlest of scent trails are used by many creatures, from fish to insects to amphibians, to pinpoint their home; and how the tiniest of songbirds are equipped for solar and magnetic orienteering over vast distances. Most movingly, Heinrich chronicles the spring return of a pair of sandhill cranes to their pond in the Alaska tundra. With his marvellously evocative prose, Heinrich portrays the psychological state of the newly arrived birds, articulating just what their yearly return truly means, to the birds and to those fortunate enough to witness this transcendently beautiful ritual.The Homing Instinct is an enchanting study of this phenomenon of the natural world, reminding us that to discount our own feelings toward home is to ignore biology itself.
Meaning and Mystery in Animal Migration
Copyright (#ulink_52c913de-cb40-588b-a976-50fc58834d1c)
William Collins
An imprint of HarperCollinsPublishers
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London SE1 9GF
WilliamCollinsBooks.com (http://www.WilliamCollinsBooks.com)
This eBook first published in Great Britain by William Collins in 2014.
First published in hardback in the US by Houghton Mifflin Harcourt 2014.
Text and illustrations copyright © Bernd Heinrich 2014
Cover photograph © Antagain / Getty Images
The author asserts his moral right to be identified as the author of this work.
A catalogue record for this book is available from the British Library.
All rights reserved under International and Pan-American Copyright Conventions. By payment of the required fees, you have been granted the non-exclusive, non-transferable right to access and read the text of this e-book on-screen. No part of this text may be reproduced, transmitted, down-loaded, decompiled, reverse engineered, or stored in or introduced into any information storage and retrieval system, in any form or by any means, whether electronic or mechanical, now known or hereinafter invented, without the express written permission of HarperCollins.
Source ISBN: 9780007594054
Ebook Edition © August 2014 ISBN: 9780007594061
Version: 2015-04-22
CONTENTS
Cover (#u86f17ac5-4aa7-5b07-bd70-520675b83c34)
Title Page (#ud15a4144-4a40-57e6-b598-73c42722fb35)
Copyright (#u33fc8821-b6c5-531d-a274-c3657b95eb72)
Preface (#u6377da0a-6478-5424-bb26-dd7bb85e3c33)
Introduction (#u33d43583-b8fa-58b8-b37d-f6f34df7cf87)
I. HOMING (#ubc69464a-d9e2-59db-be05-270a8cd4216b)
Cranes Coming Home (#u7c1fd44f-e3e8-53a0-9bec-ff448a1fb98d)
Beelining (#ue6751a10-a6c7-5cc2-98f6-31947ad20b26)
Getting to a Good Place (#uf3a156a3-dd3f-5465-b73e-02249414b001)
By the Sun, Stars, and Magnetic Compass (#uded56845-8313-5c9d-8c48-d515f8a491b5)
Smelling Their Way Home (#litres_trial_promo)
Picking the Spot (#litres_trial_promo)
II. HOME-MAKING AND MAINTAINING (#litres_trial_promo)
Architectures of Home (#litres_trial_promo)
Home-making in Suriname (#litres_trial_promo)
Home Crashers (#litres_trial_promo)
Charlotte II: A Home Within a Home (#litres_trial_promo)
The Communal Home (#litres_trial_promo)
III. HOMING IMPLICATIONS (#litres_trial_promo)
The In and Out of Boundaries (#litres_trial_promo)
Of Trees, Rocks, a Bear, and a Home (#litres_trial_promo)
On Home Ground (#litres_trial_promo)
Fire, Hearth, and Home (#litres_trial_promo)
Homing to the Herd (#litres_trial_promo)
Epilogue (#litres_trial_promo)
Further Reading (#litres_trial_promo)
Index (#litres_trial_promo)
Acknowledgments (#litres_trial_promo)
Books by Bernd Heinrich (#litres_trial_promo)
About the Publisher (#litres_trial_promo)
PREFACE (#ulink_604cb48e-a5b0-59f4-8a6b-87e3b8fb5183)
ABOUT A DECADE AGO I STARTED PULLING TOGETHER BITS and pieces on the “homing” topic and in 2011 had a book manuscript scheduled for publication. I was then living at “camp” in Maine, where I had done my fieldwork on bumblebees for years; lately my work involved feeding ravens with cow carcasses in the winter, and I then got interested in beetles that bury mouse carcasses in the summer. Soon the topic of recycling of animal carcasses of all sorts seemed more urgent than the scheduled book about getting to and living in a particular place. So, I put writing about homing on hold. By the time I again picked up my pencil, it seemed as though everything I thought of or had an interest in had, in one way or another, started to have a bearing on home and homing. In the meantime I had also been confronting personal issues of “homing,” and they seemed to take on increasingly similar forms to what I was reading about in animals.
I had already left my academic position in Vermont and wanted to return home to live in Maine, possibly in the home to which I had bonded strongly as a child. I had planted a row of trees there about thirty-five years ago. Those trees, now huge, brought back many memories related to them. They reminded me of my father, who had liked them, probably because he had strong feelings for a row of chestnut trees that also lined the way to his old home in the old country that he had often talked about. Because I had written a book about my father, and not also one about my sisters or my mother, I had come into the disfavor of both. There had been parent-offspring conflict before my mother died, and then the house stood empty. So then there was also sibling rivalry over the estate. It seemed like being in a real-life situation of the sociobiology theories, in an almost perfect rendition of a naked mole rat colony where one of the family finds a big tuber, and the others claim it as theirs, and then a vocally assertive member establishes herself as the matriarch. I realized then that the difference between what can happen to a human and to a naked mole rat family is mainly one of terminology. This thus provided the topic of home and homing a much wider perspective.
INTRODUCTION (#ulink_ee04a767-712f-5a00-a5ba-0704b3eb0255)
Our passionate preoccupation with the sky, the stars, and a God somewhere in outer space is a homing impulse. We are drawn to where we came from.
— Eric Hoffer
With all things and in all things, we are relatives.
— Native American (Sioux) proverb
I LEANED ON THE SHIP’S RAILING AT THE STERN, A TEN-YEAR-OLD boy with virtually no notion of where my family might be going. I heard the deep roar of the engines, the whine of the wind, and the rush of the churning water. I felt adrift, as though carried along like a leaf in a storm, feeling the rocking, the spray, and the endlessness and power of the waves. I had no notion that we were among multitudes who had made hard decisions to court the great unknown, or any clear idea of why my family had left the only home I’d known in a forest in Germany. The only picture of what our new home might be was that we might find magical hummingbirds, and fierce native tribes armed with knives, bows and arrows, spears, and tomahawks.
Security for me was the memory of where we had come from, specifically a little cabin in the woods and a cozy arbor of green leaves that enclosed me like a cocoon where I could see out but nobody could see in. It meant a feeling of kinship with the tiny brown wren with an upright stubby tail that sang so exuberantly near its snug feather-lined nest of green moss hidden under the upturned roots of a tree in a dark forest. I had in idle moments in my mind inhabited that nest. I found, too, the nest of an equally tiny long-tailed tit. This little bird’s home was almost invisible to the eye because it was camouflaged with lichens that matched those on the thick fork of a tall alder tree where it was placed.
The ocean all around was a spooky void. But then, after several days at sea, a huge white bird with a black back appeared as if out of nowhere, and it followed us closely. I saw its dark expressionless eyes scanning us. It was an albatross. It skimmed close over the waves and sometimes lifted above them, circled back, and then picked up momentum to again skim alongside our boat. It followed us for hours, maybe even days.
The albatross was big and flew without beating its wings. Years later I wondered if, even in the featureless open ocean where so much looked the same every hour and every day, it may have known where it was all along. How do we find our home and recognize it when we find it? These questions were inchoate then, but given the examples of other animals, they put many ideas of home and homing in context.
Later, as a graduate student, I read that pigeons could return home to their loft even when released in unfamiliar territory, and that some other birds could navigate continental distances using the sun and the stars. There were few answers to how they did it. But I read about researchers at Cornell University who attached magnets to the heads of pigeons and got them all confused. Donald Griffin, my scientific hero (who had discovered how bats can snatch silent moths out of the air in a totally dark room that had wires strung all over the place), was releasing seagulls over forest where they could never have been before and then tracking the birds’ flight paths by following them in an airplane. Most of his birds turned in circles before some of them flew straight, although why was not clear. Searching for a thesis problem to work on, I wrote to ask him if birds passing through clouds might keep in a straight line by listening to the calls flocks make while migrating. He replied in a long, thoughtful letter to let me know that this idea was too simplistic, and that one should not discount much more complicated mechanisms. That was excellent advice. I did not then have the means to solve any of these puzzles, but over the years I have kept in touch with the evolving field of animal navigation and its relevance to the need for a home.
For other animals and for us, home is a “nest” where we live, where our young are reared. It is also the surrounding territory that supports us. “Homing” is migrating to and identifying a suitable area for living and reproducing and making it fit our needs, and the orienting and ability to return to our own good place if we are displaced from it. Homing is highly specific for each species, yet similarly relevant to most animals. And the exceptions are illuminated by the rule.
The image of that albatross took on more meaning decades later, after I learned that the species mates for life and returns to the same pinpoint of its home, on some island shore where it was born, perhaps fifteen hundred kilometers distant. During the years when it grows to adulthood it may never be in sight of land. Seven to ten years after having left its home, it returns there to nest. It chooses to go there because of its bond. When a pair eventually have a chick in a nest of their own, each parent may travel over fifteen hundred kilometers of ocean to find a single big meal of squid, and after gathering up a full crop, it then flies home in a direct line; it knows where it is at all times.
The broad topic of homing subsumes many biological disciplines. In order to show the connections among all animals and us, I have interpreted the traditional use of an animal’s “territory,” or “home territory,” simply as “home.” We think of “home” primarily as a dwelling, but in order to be inclusive with other animals, I here consider their dwellings to be their homes as well. My application of the same terms to different species is deliberate for the sake of scientific rigor and objectivity, to acknowledge the continuity between our lives and those of the rest of life. I realize that this smacks to some of anthropomorphism, a pejorative term that has been used for the purpose of separating us from the rest of life. The behaviors involved in homing include drives, emotions, and to some extent also reason.
A home makes many animals’ lives possible: home is life-giving and sought after with a passion to have and hold. We humans are not thinking much about “home” for animals when we confine them in cages devoid of almost everything they need except air, food, and water in a dispenser, or when we destroy the habitat that contains the essentials of home for many species. So I begin our exploration of home and its implications with the example of the common loons, Gavia immer, birds that may live for decades. The collaborative study by three biologists, Walter Piper, Jay Mager, and Charles Walcott, reveals how important home can be — enough for fights to the death.
Loons spend winters in the open ocean, but a pair migrate from it and across the land back to their home, a specific northern pond or lake, to nest along its shore in the spring and raise one or two chicks out on the water. Starting almost immediately after ice-out and almost until freeze-up, camp owners along a lake routinely see “their” pair of loons year after year. It had long been assumed that the same individuals return each year and live as monogamous pairs on their strongly defended home territory. Huge surprises were in store after 1992, when techniques (using a boat, a strong light, and a net) were developed to capture loons and mark them with colored leg bands to identify individuals. In a long-term study of a population of loons in Wisconsin in a cluster of about a hundred lakes, it turned out that a pair of loons indeed returned year after year to their home. However, they were not always the same birds. As expected, given their longevity and reproductive potential, there were many “floaters,” those still without a home, and some of them routinely replaced members of a pair.
The floaters regularly visited different pairs at home at their respective lakes, and spirited vocal meetings resulted. These seldom led to fights, but they were not just friendly visits. These floaters were at first thought to invade others’ home grounds in order to make “extra-pair parenting” attempts (which in males refers to extra-pair copulations and in females to egg dumping into the others’ nests). However, DNA fingerprinting of the young loons from four dozen families produced not a single incident of extra-pair parenting. Instead, the visits by floaters were of an entirely different nature. They had an almost literally “deadly” purpose. The floaters were scouting — making assessments of both the worthiness of the others’ real estate and the defensive capabilities of the resident males — to gauge the possibility for future takeovers.
Loons nest on the ground along shorelines, if islands are not available. Shorelines, if low, are risky nesting places, not only because of potential flooding in early spring, but also because they are within easy reach of raccoons, skunks, and other predators. Most birds test what is a “good home” by direct experience — success in raising a brood there. Or, like the loons, they assess the experience of others: whether or not chicks have been raised there. So, if the territory does yield young, a floater, who apparently finds this out through scouting, may risk a fight in an attempted takeover to remove the defending male (if he achieves takeover, he automatically gets to keep the female who remains on her home). But the floater can risk an attack only when he is about four or five years old. When he is in prime physical condition and has much to gain — namely, a potential lifetime of home ownership — it makes sense to risk the battle. The older territory owner might then fight to the death, presumably because he has much to lose, and almost no chance to gain another home.
Loons may seem extreme in the lengths to which they go to secure a home, as do other birds that risk the hazards of migrating thousands of kilometers. Yet, in the movie I watched with rapt attention on board ship on the way to America at age ten, people on ponies shot arrows at others on a wagon train. All were emotionally charged, because each was fighting for something sacred, and therefore each was willing to risk his life, for defending or wanting a home.
We have learned much about thousands of animal species that twice annually risk their lives to migrate to an exact pinpoint, such as an oceanic island in the case of the albatross or a pond in the vastness of a continent in the loon’s. They open a window with a broad view onto our unending quest into the mysterious minds of animals, and in the process they illuminate our own. How does one tie the vastness that includes other animals, and so much that affects us personally and socially, together into a story?
Writing this book reminded me of when, after riding more or less unconscious in the slipstream of history for over fifty years, I started putting down stones chosen from a vast array of differently shaped fieldstones to build a house foundation. I couldn’t chop them to size or knock the edges off so that they would fit into the inevitable empty spaces to make neat connections. Nor did I want to shape them, like bricks, to make a tidy but artificial structure. The stones found in nature, like facts, are endlessly numerous, wild, and complex. As the famous British geneticist and evolutionary biologist J.B.S. Haldane quipped, “My own suspicion is that the universe is not only queerer than we suppose, but queerer than we can suppose.” I hope to give here a view of some of the “stones” of homing, and their origins, and how they apply in real life.
Homing is central to many aspects of our and other animals’ lives. To understand the meaning of home, like any other phenomenon, it helps to step back and see from another’s world. Animals give more than just clues to the why and the how of homing. They show what is possible, what has been tested, and what has worked over millions of years of evolution.
In this book I cannot hope to provide an in-depth treatment of any issue. My viewpoint is wide, and admittedly personal. Thousands of scientific references are possible for any one topic, and I am not an expert on any topic. Those references I cite are in no way meant to be the specific last word on the topic. They tend to be those I am most familiar with. I apologize for all of the amazing stories and all the details that I have left out. I have tried to speculate freely, and I hope that this will open discussion, not close it.
PART I (#ulink_9a145561-ebcf-5689-ae07-6d273cd92811)
Homing (#ulink_9a145561-ebcf-5689-ae07-6d273cd92811)
A skull I have lying on my desk is as big around as a somewhat flattened coffee mug, but it comes to a point at the front and has two large eye sockets. The bone on the top of the head is sculpted in ridges and furrows and looks like weathered stone. Seen from the back, the skull has a backward-opening hole on each side. The holes anchored the animal’s powerful jaw muscles. Between them is a much smaller hole. I am looking through this hole (foramen magnum) for the brain cavity, but this large head contains scarcely any brain at all, just an extension of the dorsal nerve cord that runs up from the neck.
It’s the skull of a snapping turtle, a female who met her end as she was crossing the road to dig a hole in the gravel to lay her clutch of eggs. As in previous years, she had come from her bog a mere hundred meters away.
The local snapping turtle is no great wanderer. But two sea turtles — the green turtle, Chelonia mydas, and the leatherback, Dermochelys coriacea — are renowned for migrations spanning entire oceans. The leatherback as an adult feeds in northern cold waters but nests on tropical beaches. It can weigh up to a ton, and although the skull of one that I had the opportunity to examine was as big as a basketball, the cavity holding its brain was, like the snapping turtle’s, only a slight expansion at the end of the vertebral column. It could barely have held a walnut. The green’s could have held two hazelnuts. Their skulls hardly differ from that of any turtle, whether of a species alive now or one that lived 215 million years ago, at a time at least three times more distant than that of the last dinosaurs.
The minute dimensions of some animals’ brains are as astounding as the homing capacity of some of their owners. Like albatrosses, sea turtles of various species lay their eggs in colonies with others of their kind on specific ocean beaches. After the young hatch from the eggs buried in the sand, they head for the water and spend years at sea. They may travel thousands of kilometers, and then, a decade or two later when they are ready to lay their eggs, they return to their birthplace. They mate in the water nearby, and the females then come ashore to dig their nest holes in the sand and to drop in their eggs. How are they able to find their old home after years of wandering in the vastness of the oceans, when we, if taken blindfolded to and then released in unfamiliar woods, would, despite our highly sophisticated massive brains, be as likely to head off in a wrong as a right direction? To get around in unknown territory most of us need a map with which to find at least one known fixed feature that we can both see on the ground and locate on that map, and a compass.
What knowledge and what kind of urges does it take for some birds to fly nonstop for nearly ten thousand kilometers, spending all day and night on the wing, until their body weight halves as they not only burn up all of their body’s food stores but even sacrifice muscle, digestive tract, and other entrails — almost everything except their brains?
CRANES COMING HOME (#ulink_ad57f3f7-2050-5a0d-94c3-b1c3f076fd7c)
If feeling fails you, vain will be your course.
— Johann Wolfgang von Goethe, Faust
MILLIE AND ROY ARE A PAIR OF SANDHILL CRANES THAT STAY for most of the year in Texas or Mexico but travel north in April and have for at least fifteen years nested and raised their one or two offspring, known as colts, in a small bog in the Goldstream Valley near Fairbanks, Alaska. Their home is adjacent to the home of my friends George Happ and his wife, Christy Yuncker. George was an insect physiologist and chairman of the Zoology Department at the University of Vermont where I was hired in 1980, and he later moved to the University of Alaska and the land of the Iditarod, where the two built their home in the wild land near Fairbanks. They invited me to visit them and “their” cranes, and I was eager to do it.
The thousands of square kilometers of central Alaska’s permafrost-covered taiga consist of stunted blue-green spruce and white birch, with a groundcover of green-yellow moss and twiggy Labrador tea whose evergreen leaves curl at the edges and have a soft beige fuzz on their undersides. Chalky lichen and small shiny cranberry leaves decorate a thin black soil overlying the permafrost that can extend thirty meters down. In this expanse, there are many bogs or pingos, which are the result of an ice dome (groundwater that freezes into an upwardly bulging ice lens) that has melted and created a depression where a pond or a lake is then formed. After a few centuries, a floating mat of vegetation grows in from the edges to create a floating bog. Such pingo bogs have become the favorite home sites of sandhill cranes.
Portrait of Millie and Roy
George and Christy’s pingo in the Goldstream Valley is, like others, clear of trees but surrounded by stunted black spruces. It is the home site not only for the crane pair but also for Bonaparte’s and mew gulls, pintail and mallard ducks, and sometimes horned grebes. I intended to arrive several days in advance of the cranes’ anticipated return, to try to watch their homecoming. Surely this return during the first week in May would be a big event in their lives, and I wanted to see their reactions to their old home.
Millie and Roy had last been seen as they left the bog in the previous year, on September 11, 2008, for their southward migration. They had been delayed from their normal end-of-August departure date because Oblio, their colt, had a leg injury that prevented him from being ready in time for the family flight to western Texas. Waiting for him saved his life; we know that young cranes, as well as geese and swans, learn the route between wintering and breeding homes from their parents. The proof and the implications of the necessity of the young to be able to follow their parents, or alloparents, in order to migrate were perhaps most convincingly demonstrated by William Lishman after he first played parent to hand-raised geese that he later led as a flock with an ultralight aircraft. He also led sandhills in this way. Finally he led a flock of whooping cranes from their breeding grounds in Wisconsin to establish new homes for them in Florida. However, nobody as far as I know has been able to follow wild birds, and my chances of seeing Millie and Roy touch down for the first time on their arrival this year might be slim. But I felt it was worth a try.
Cranes, like other large birds, grow slowly. It takes them thirty to thirty-two days to incubate their two eggs, and another fifty-five days for their (usually one) colt to be able to fly well enough to migrate. This far north there is only a narrow window of time for cranes to breed successfully, especially for those that fly even farther to breed, as some of those wintering in Mexico do, in Siberia. If late in arriving, they waste their effort of migrating the thousands of kilometers north. If they are too early, snow and ice cover all food sources. This year had been a winter of heavy snows in central Alaska. Even the boreal owls were starving from their inability to reach the voles under the snow. By late April, when I arrived, the woods around Millie and Roy’s home bog were still under at least half a meter of snow, and the cranes had not yet shown up.
I would have liked to fly with the cranes on their homeward journey, but the best I could do, apart from trying to beat them to their destination, was to see a piece of their flight path. My transcontinental flight of 3,872 kilometers from JFK Airport in New York was followed by a direct flight on April 23 from Seattle to Fairbanks, and I spent most of the three and a half hours of the 2,467-kilometer flight from Seattle north in the Boeing 737–800 with my face pressed to the window, trying to see like a crane. How did the cranes navigate and negotiate their five-thousand-kilometer journey from Texas or Mexico to come home to their own pingo out of thousands of others scattered throughout the vast and seemingly unending Alaskan taiga?
The cranes arrive lean at their main staging area, at the Platte River in Nebraska, and stay three weeks to gather reserves for their continuing journey north. When ready, they gather with thousands of others and wheel high in the sky into giant “chimneys,” to travel together on their common journey. Once in Alaska, they take separate paths to their individual homes, and a third of them fly beyond, to their homes in Siberia.
We had scarcely lifted off in Seattle when we passed over white-capped mountains with knife-edged ridges, dark forested valleys, and peninsulas surrounded by blue-gray water. An hour later, cruising at about eight hundred kilometers per hour at eleven thousand meters, there was ever more of the same — white mountains as far as the eye could see. Another hour — it was still the same. To me, barely a feature stood out from the jumble of endless peaks that melded into each other, and the vast mountain scape was broken only by frozen lakes glinting in the evening light. And so it continued for yet another hour. When we started our descent to Fairbanks, I saw oxbows of meandering rivers, and finally the thin thread of a road.
Cranes, swans, and geese travel south in the fall as family groups. On their way, the young learn the route they will later take north in the spring, to come back to try to settle near where they were born. What they see and remember seems astounding. I might, with intense concentration, memorize a tiny portion of the way, perhaps around this or over that mountain. But these cranes come not from my point of departure, the state of Washington, but from considerably farther south. (Four cranes from the Coldstream Valley that the Alaska Department of Fish and Game had equipped with radio transmitters ended up in various parts of Texas in the winter.) I could never retrace even my own much shorter flight route from Seattle, even if I were to return the day after having flown over it, much less a half-year later. What are the cognitive mechanisms that allow the birds to do this?
Day after day for almost eight months now there had been no crane at the pingo. For most of that time the ground had been under a deep layer of snow that locked any food out of reach. What would happen if, after their long-distance flying, the pair were to arrive at their home and find the bog still under snow and ice with no cranberries to be found and no voles to catch? How much can cranes afford to gamble in order to try to come on time, or even early?
It was only in the last week of April, after another snowstorm, that the weather suddenly warmed, and just then, on the 24th of April, on my first morning, we heard a crane in the distance. Still, no cranes landed on the pingo on the 25th, 26th, 27th. But the next morning at dawn I awoke to the loud and penetrating trumpeting calls of a single crane. These metallic sounds are unearthly; as Aldo Leopold wrote in his “Marshland Elegy,” they evoke “wildness incarnate.” On and on this bird shattered the dawn’s stillness, and I ran out to look. But the bird was then distant, and the sound kept shifting position, so I presumed it was flying around in great circles, possibly looking for a patch of cleared ground; the mossy floor of the nearby stunted spruce forest was still covered in deep snow.
That evening we sat down to supper by the window facing the wide-open panorama of the pingo in front of us. The sun was still high. We were just polishing off the last of our freshly grilled salmon when we looked up to see a crane with spread wings gliding down for a landing. Its long thin legs touched down gently on the still-thick ice of the pond. It bugled and sprang up half a meter or so, unfurling its long neck with beak held skyward and with extended wings at the apogee of its graceful leap as if to catch itself in the air to prolong this moment. It looked like a physical embodiment of joy and excitement. The crane kept leaping, all the while continuing its stirring bugling. Cranes don’t do this every time they land; this was indeed a special landing. Finishing its dance, the crane started to walk in a contrastingly slow and deliberate manner, thrusting its head forward and up with each step, and at the same time opening its bill and making a very different, trilling, call.
The crane walked and leaped in several more repetitions of its dance before eventually lifting off with even wing beats to sail off in the same direction from which it had come, its haunting cries growing faint in the distance. Two hours later the (same?) crane came again, but this time it circled the bog only once before leaving, continuing its calling. It had given the impression that it was glad to be back but was at the same time agitated and looking for something; a mate? In previous years Millie and Roy had always returned as a couple, George and Christy told me, so lacking positive identification, we were skeptical that this was either of them.
I had barely gone to bed that night when I heard another one or possibly two cranes calling excitedly, while a third seemed to answer from the distance. I jumped up and rushed outside to look: a pair were walking from one end of the bog to the other. George and Christy were up also, and for the next hour, until it got dark at 11:15 p.m., we watched. One of the cranes stood tall and extended his head and neck forward and up, reminding me of the dominance display of a male raven. This was Roy. The second one, whom I would soon identify by her walk and talk and narrower white face patch, held her neck and head in a more downward curve like a heron’s, projecting her head slightly forward with each step. She started to pick up cattail fronds and grasses and then to deposit them, sitting down briefly on the materials. Was she encouraging her mate by suggesting to him that it was time to start nesting at one or another of these potential nest sites? I could hardly wait to see what would happen next.
Crane pair coming home
Another clamor awoke me in the early dawn, around 4:00 a.m. There had been a heavy overnight frost and the two cranes were standing on the ice in the middle of the bog. Both Millie and Roy tossed their heads up in a quick motion, their bills opening wide during each call. We heard what sounded like a hammer hitting a metal bell or drum, as he opened his bill once to make each call and she chimed in at the same time but called and opened her bill twice to make two short similar cries of a higher pitch. It was a composite call made by both together; a duet. A third, distant crane responded. The distant calls, and the pair’s duet, were repeated back and forth, often and loudly.
The vigor of the pair’s unison calling was still palpable, even when the first morning light lit the sky and silhouetted the black spruces, and when I thought about the enormous effort they had invested to get here, I realized what was at stake: home ownership. The pair’s loud clanging calls attracted no others flying in from the distance; instead, the calls are a vocal “no trespassing” sign, one leaving no doubt that any potential challenger would be facing not just one bird, but a united, cooperating pair.
I watched the pair for another hour. She by then occasionally fluffed herself out and, as she had done the day before, continued periodically to squat where she had pulled at or dropped sedge and other potential nesting material.
The pair continued their slow, deliberate steps that morning while meandering from one end of the bog to the other, as though inspecting every square centimeter of it, and at 9:00 p.m. we saw Roy jump high and with outstretched wings dance by himself out on the ice. Millie dashed by him with fluttering wings, to round out a mutual performance. A little later we heard a purring call as she spread her wings to the sides and stood still. Roy, with outstretched neck and elevated bill, jumped onto her back and, balancing himself with a few wing beats, mated. He dismounted after a couple of seconds. Both then bowed to each other and continued their walk. They were home, intended to nest, and had now sealed the deal.
Still, the lone bird that had tried to intrude on their turf did not easily abandon its intended claim. But why would this crane want a claim without a mate to nest with? Was it Oblio, their grown colt from last summer? Was he, now that Millie and Roy were re-nesting, finally being “thrown out of the nest” by his parents, who no longer tolerated company of any sort? I suspect this was the case. The offspring of most birds have a strong attachment to home. This emotion is a biologically relevant drive, because home is where reproduction has proven to be successful. But the young have a lifetime ahead of them, and if for some reason the parents don’t make it back home, the offspring could inherit their territory. If the parents do come back to reclaim their home, at least nearby territory would be more like the old home than the far-off unknown. Even if a bird is without a mate, finding a suitable territory is often a prerequisite to getting one.
Shortly after we had breakfast the next morning, the lone crane flew over once again, and the pair immediately launched their synchronized duet as a vocal challenge. This time, although the lone crane did not land, the pair jumped into the air and chased it until all three were out of our sight and hearing. But the pair returned soon and then again performed several nest-building probes. They again mated in what would become a routine for the next several days: at least once in the morning and once in late afternoon or at night.
When I first saw the cranes the next dawn, each was standing on one leg with its head tucked into its back feathers. Ice had again solidly covered their pond, after having partially melted along the edges the day before. Both Millie and Roy seemed to be asleep, although he, balancing himself on one leg, occasionally reached up with the other to scratch his chin and head with the toes of the foot. But whenever a crane called from the distance, both their heads shot up instantly, and they renewed their spirited in-unison call, she making the two short notes and her mate making one, at a lower pitch.
We had so far not seen them feed. Indeed, it was hard to imagine that there was food available in any case. If lucky, they might by now catch a vole or find a few of last year’s cranberries, but this year that would probably not be likely until days later when the snow would melt. A crane’s large body size requires much more food than a small bird’s, but that same large size is an advantage in tiding them over during lean periods, and thus to return to their homes before the anticipated flush of food becomes available.
As it got lighter on this dawn, the cranes soon had company. A pair of swans and then a small flock of five Canada geese flew by. A robin sang. By about 7:00 a.m., the crane pair became animated as well, walking over the bog while picking here and there, and again mating.
We did not see the pair most of that afternoon. The lone crane, seemingly to take advantage of their absence, again flew in and this time landed, looked around, and repeatedly made a trilling call. But in about fifteen seconds the pair flew in as if out of nowhere, and one of them sped over the ice as if to attack the interloper, which immediately flew off. The pair gave chase, and all three disappeared from sight. In several minutes the single bird returned. Again the pair came and caught up to the lone crane before it had a chance to take off, and this time they attacked it viciously, in a flurry of flailing wings.
The pair had by now, after the third day, won the major part of the battle. Barring accidents, they would within days lay their two eggs and go on to raise their colt. Normally both eggs hatch, but as in some eagles and vultures, usually only one chick survives, probably because one gets fed less and then weakens and eventually starves. Presumably through evolutionary history, for the fast growth required to reach full development and readiness to migrate by August, there has not been enough food to raise two colts at once. One might suppose the cranes could simply lay only one egg, but sometimes an egg does not hatch, and the second is insurance.
The pair seemed more animated after their last fight with the lone intruder, and by evening they again mated (for at least the third time that day). In most birds one mating is enough to fertilize the eggs. Perhaps several matings are insurance, but this seemed more than enough for insurance. Perhaps, like their dances, mating is additionally part of their bonding ritual.
A pair of mallards, and then a pair of pintails, arrived in the evening, and the ducks swam next to each other near the cranes at the edge of the pond, where some open water had reappeared during the day. The cranes ignored them and again walked in their stately manner back and forth across the ice of the pond, and now they pecked in the low vegetation being exposed along the edges. They were by now finding overwintered cranberries exposed by the melting snow.
On the evening before I would leave for my journey home, Christy and George hosted a potluck party. Shadows fell on the white frozen middle of the pingo as the western sky turned yellow and orange and the spruces became dark silhouettes. A pair of pintails again landed in the open water along the pond’s edge. The cranes were standing, each on one leg, their heads tucked into their back feathers. People crowded around the spotting scope in the living room, watching them occasionally shift position, lower a leg, poke a head out to look around. Suddenly the person then at the scope erupted with an exclamation: “They are mating!” She had seen the male approach the female with her spread wings, mount, flutter, and jump off. The pair had bowed to each other. Suddenly many people crowded around the scope to watch.
Why, I wondered, would anyone, or almost everyone, want to watch cranes mate? Why was nobody interested in watching the mating activity of the two ducks, or of the numerous redpolls? Could it be, I wondered, because we feel a closer kinship with cranes than with other birds?
Cranes are similar to us in many ways. Some are nearly as tall as a person. They walk on two long legs like us, albeit with a much more graceful and deliberate gait, so that they remind one of a caricature of a gentleman or an elegant woman on a leisurely stroll. The sandhill crane’s red bald pate and sharp yellow eye add to the caricature. Cranes form lifelong pairs and stay together as families, but they are also gregarious and join up into large groups. They form a strong attachment to their home. They not only make music with trumpeting calls that sound like bugles, but they also dance, and do so on various occasions.
All of the fourteen species of the world’s cranes dance. Crane dancing involves running, leaping into the air, flapping the wings, turning in circles, stiff-legged walking, bowing, stopping and starting, pirouetting, and even throwing sticks. Dancing is primarily done by pairs and presumably functions in cementing pair bonds and/or synchronizing reproduction. But it can also be induced at any time, and it stimulates other cranes to dance. Even the young colts perform some of the species’ dance. Possibly it serves as practice and could be motivated by the same basic emotions of joy that are an indicator of health important to mating.
Cranes’ dances often stimulate humans to dance as well and have been mimicked in many cultures all over the world where cranes live. Crane dances were performed by ancient Chinese, Japanese, southern African, and Siberian people. If not emulated, cranes are admired. In the Blackfoot tribe of Native Americans of northern Montana, the last name “Running Crane” is common.
Nerissa Russell, an anthropologist, and Kevin McGowan, an ornithologist from Cornell University, revealed that eighty-five hundred years ago at a Neolithic site in what is now Turkey, people probably performed crane dances using crane wings as props that were laced to the arms. Furthermore, someone of these people apparently hid a single crane wing in a narrow space in the wall of a mud-brick house along with other special objects (a cattle horn, goat horns, a dog head, and a stone mace head). Russell and McGowan also found evidence that vultures may have been hunted for their feathers for presumably a much different costume worn as well for a ceremonial purpose. The authors inferred that the cranes were linked with happiness, vitality, fertility, and renewal (since they arrived in the spring). While the crane dance was one of life and birth, and possibly marriage and rebirth, the vulture dance was associated with death and perhaps return to the afterlife.
Russell and McGowan believe that the crane wing interred in the wall of the house was never intended to be seen. It was a symbolic object related to marriage and construction of a new home and may have been coincident with a particular human marriage and home-making. The associations among dancing, pairing, and raising young and home would have been natural for people who saw cranes return to their home ground, just as I had seen Millie and Roy do. Seeing the close parallels in the biology of the birds with their own lives, and understanding the cranes’ dancing as helping to make or cause the good things that followed, Neolithic people would have been compelled to symbolically emulate the crane dance of homecoming and of new life.
BEELINING (#ulink_023ec351-cb60-54c0-8f4c-006a10f6452b)
Observation sets the problem; experiment solves it, always presuming that it can be solved.
— Jean-Henri Fabre
CRANES FLY AN ENORMOUS DISTANCE TWICE ANNUALLY, BUT relative to their size, bees also fly huge distances — up to ten kilometers — and the foragers may perform such trips hourly. We can experiment with them to find out how they navigate. What we know about bee homing so far is nothing less than astounding, and it is built on a long history of research, primarily pioneered by the imaginative experiments dreamed up and performed by an Austrian named Karl von Frisch and his colleagues that date back over a half-century. Arguably, our knowledge dates back still further to early American frontiersmen trying to find bees’ treasure troves of honey.
In 1782, Hector St. John Crèvecoeur, a writer and farmer from Orange County in New York State, wrote:
After I have done sowing, by way of recreation, I prepare for a week’s jaunt in the woods, not to hunt either the deer or the bear, as my neighbors do, but to catch the more harmless bees … I proceed to such woods as one at a distance from any settlements. I carefully examine whether they abound in large trees, if so, I make a small fire on some flat stones, in a convenient place; on the fire I put some wax; close by this fire, on another stove, I drop honey in distinct drops, which I surround with small quantities of vermillion, laid on the stones; and I retire carefully to watch whether any bees appear. If there are any in the neighborhood, I rest assured that the smell of burnt wax will unavoidably attract them; they will find the honey, for they are fond of preying on that which is not their own; and in their approach they will necessarily tinge themselves with some particles of vermillion, which will adhere long to their bodies. I next fix my compass, to find out their course — and, by the assistance of my watch, I observe how long those are returning which are marked with vermillion. Thus possessed of the course, and, in some measure the distance, which I can easily guess at, I follow the first, and seldom fail of coming to the tree where those republics are lodged. I then mark it [presumably with his name to claim ownership].
James Fenimore Cooper, author of the Leatherstocking Tales of the American frontier, of which The Last of the Mohicans is probably best known, in 1848 published the novel The Oak-Openings; or, The Bee-Hunter. Here Cooper depicts a different, perhaps more reliable method than Crèvecoeur’s of the frontier activity that came to be called “beelining.” Cooper’s story takes place during July 1812, in the “unpeopled forest of Michigan,” where, due to the Native Americans’ lighting periodic fires to clear the ground, there were many flowers among the scattered oaks. This was ideal honeybee habitat, and here the bee hunter Benjamin Boden, nicknamed “Ben Buzz,” practices his art. Ben captures a bee from a flower by placing a glass tumbler over it and sliding his hand underneath. He then places the tumbler with the captured bee on a stump next to a piece of filled honeycomb. He puts his hat over the tumbler and the honeycomb so the bee will not be able to escape. He waits as the bee, stumbling around in the dark, eventually finds the honey. Once it is preoccupied with imbibing the honey, it quits buzzing, and the silence is the signal for Ben to remove the hat and then the glass, as the bee will stay to finish its feast and will fly up, circle the honeycomb, and depart directly toward its nest. He then follows the bee to the tree, chops it down, and is rewarded with just over one hundred kilograms of honey. Easier said than done.
American honey hunters eventually added refinements to their beelining techniques. The main improvement was the invention and use of a “bee box,” a small wooden box designed to catch a bee and get it “drunk” on a hunk of honeycomb. It was used in Maine when I was a kid (I still own mine). George Harold Edgell, a lifelong bee tree hunter from New Hampshire, wrote in 1949 in a pamphlet titled The Bee Hunter that “one’s first task is to catch a bee and paint its tail blue” and “this must be done gently [because] bees do not like to be painted. To paint a bee, it is best to wait until it is eagerly sucking up a thick sugar syrup and is too pre-occupied to notice.”
By 1901 Maurice Maeterlinck, the Belgian playwright and Nobel laureate in literature, described in The Life of the Bee his scientific experiments on bees that were individually identified with daubs of paint, from which he deduced that these insects could communicate their discoveries of food bonanzas to hive mates that would then navigate directly to the food. However, American woodsmen not only had used similar methods, but had also, through their beelining, already gleaned that same surprising insight into what the bees could do. Maeterlinck credited his American predecessors for their discoveries and wrote, “The possession of this faculty [to communicate food locations to hive mates that then can navigate to the food] is so well known to American bee hunters that they trade upon it when engaged in searching for nests.”
Although early American woodsmen, whose lives depended almost directly on the knowledge gained by close contact with nature, were beelining devotees who had deduced that honeybees recruit hive mates, it would remain for Karl von Frisch to unravel the marvelous story of how the bees communicate within the hive. He earned the Nobel Prize in Physiology or Medicine for this work. I feel lucky that a Maine neighbor, Floyd Adams, took me beelining when I was eleven years old, and that when I was a teenager, my father gave me an inspiring little book by von Frisch entitled Bees: Their Vision, Chemical Senses, and Language. It explained the experiments that he and colleagues had performed. They were mesmerizing because they connected the practical experience of beelining in the Maine woods with the imaginative power of a scientist who had penetrated into the core of the bees’ world, their hive, their home.
Floyd’s family’s home was the farm four hundred meters down our dirt road. It was populated by chickens, geese, cows, pigs, plus all the other usual and unusual wildlife that lives in a place with a tolerance for disorder. Along with Floyd, my companions were the four Adams boys, Butchy, Billy, Jimmy, and Robert, an in-law of theirs. Floyd, a dark-haired, mustachioed, wounded Marine Corps veteran recently returned from the Pacific, had a bad limp and a thirst for Black Label beer. Leona, his blond, petite wife, appreciated his fondness for honey but less so his taste for beer. He and the “boys,” after a hot day haying, sometimes went fishing on our nearby Pease Pond in the evening, but in August our big draw was always the beelining.
After we found a bee tree, we carved our initials into the bark to proclaim ownership (property lines were irrelevant with regard to bee trees; finders keepers was the rule), and at some convenient time we returned with crosscut saw, axes, wedges, a beehive, and pails and kettles for honey. Getting part of our living from the land was fun, and it meant understanding and using the bees’ homing behavior to find their hollow trees in the forest and resettling them into a new home, which we brought back to the farm and set up at a window in the attic of the house.
Fast-forward to a quarter-century later: My nephew Charlie Sewall and I are in a patch of goldenrod blooming in a pasture where each fall the wild honeybees gather nectar to top off their honey stores for the coming winter. We start by capturing a single bee in our bee box, a simple four-sided wooden box that has a ten-by-fifteen-centimeter piece of honeycomb with sugar syrup filling out the bottom. We dab the box with a drop of anise for scent and capture our bee by holding the box under her after she has landed on a flower and then slapping the box cover over her. At first the captive buzzes in the box trying to escape, but the buzzing stops when she stumbles onto the sugar syrup and starts to tank up, which will take her a minute or two. We then remove the cover and set the open box onto a pole that reaches to just above the tips of the goldenrod. We gently daub her with a spot of paint while she is absorbed in sucking up syrup, as I remembered Floyd doing. We then hunker down into the goldenrod and wait as she continues sucking up her newfound sweets that she will soon share with her hive mates. After about two minutes, her honey stomach is filled. She crawls out onto the edge of the box, stops to wipe her antennae with her front feet, lifts off, and flies back and forth downwind of the box. We duck lower to keep her silhouetted in sight against the sky as she starts flying loops, which become increasingly wider and oriented in one direction. Finally she straightens her flight path and takes off, making a “beeline” into the distance. Knowing that nobody in that direction keeps bees, it’s clear that she is on her way to a bee tree. She will soon be back with others, and we then consult our wristwatches to time her trip. A bee flies about four hundred meters a minute, and it may take her three to six minutes in the hive to regurgitate and unload her honey stomach’s contents into the mouths of begging, receiving bees.
We settle down and wait, and after perhaps ten minutes or less a bee suddenly appears and makes very rapid zigzagging flights just downwind of the box. The sound of her fight has a higher pitch than that of the bees foraging on the nearby goldenrod flowers. This means that she is more motivated and has a higher body temperature because of the rich food she is expecting. She settles into the box and starts imbibing the syrup. More bees will come soon, and when they get near our bee box, they will be guided in by the scent of the anise that marks the spot. After they tank up, we watch their flight directions.
If the food is in the immediate home vicinity, the bee does a “round dance” on the honeycomb when she returns to her home. She repeatedly runs in small circles while shaking her abdomen, and she regurgitates small samples of her find at intervals during her dance. If they become motivated after receiving information about the quality and scent of the food advertised, her hive mates leave the hive and search for the advertised food. If it is beyond a few hundred meters, the bee alters her dance to also contain information concerning location. The distance of the journey to the food is proportional to the duration of the waggle runs, and the angle of the straight runs with respect to the vertical direction informs the bees in what direction to fly when they leave the hive. If the straight run is in the up-direction on the honeycombs (which always hang vertically in the hive), the food source is in the direction toward the sun. If the food location is, for example, at an angle of ten degrees to the right of the vertical, the food direction is ten degrees to the right of the horizontal component of the sun direction when the bee would fly from the hive. Thus, her behavior is a symbolic representation in body movements of the flight to the food.
The first steps in the evolution of recruitment likely involved simple alerting signals in or at the nest entrance before takeoff. Other bees could have followed those signaling bees, probably by scent, for at least a short distance in flight. Through a few million years, the alerting likely became modified to take on an ever-greater leading function by bees flying in an ever more conspicuous manner in the direction of the food, so that followers could start off flying with ever greater accuracy in the right direction. These flights, later in the evolutionary progression, were eventually restricted to a buzz run directly on the top of the combs, but still in the food direction. We can infer this, because such “primitive” recruitment is still found in some tropical honeybee species that have their combs in the open air, where this mechanism makes sense. But open-air homes, though convenient for such communication of food location, were vulnerable to predators and also precluded the bees from living in huge areas of the globe, those with cold climates.
Homes in hollow trees allowed the bees to live in areas where they would otherwise be excluded because of cold and/or nest predators. But in such safer homes the combs hung from the roof of a cavity and left no horizontal dancing platform, and additionally the “dance floor” was now in darkness, so bees could not point directly toward the food. Even if they could, they would not be seen. But a breakthrough for indicating horizontal directions on vertical surfaces became possible after some bees started using the hanging flat surfaces of combs as their dancing platform while indicating the sun’s location as the up or “toward” direction in their dance. Additionally, tactile rather than visual orientation became predominant for recruits in reading the code within the nest.
It is amazing enough for an animal to be able to navigate to a location it has never been to before. But some ants do something even more amazing. In North Africa, desert ants live in underground homes where they are protected from the heat. But they must venture out onto the searing surface periodically to forage by scavenging on heat-killed prey. The ants are fast runners that have evolved a very high tolerance for heat. Still, at times it is a matter of life and death even for them to make it back to their cool underground home; they cannot afford to wander on the sand surface for an extended time without access to their shelter to cool down and replenish body fluids. This is where their homing ability comes in; they may have zigzagged in all directions to find a heat-killed insect, but after finding one they must make a straight “ant line” directly back home. This begs the question, Since they are often on a featureless plain and have not kept a steady course, how do they know in what direction to head home?
If one captures bees in one pasture and releases them in another, they usually depart in the direction they would have flown from the original field. That is, they act as one would expect if they do not realize that they have been moved to a new location. Rüdiger Wehner and his colleagues at the University of Zurich came to the same conclusion about desert ant homing in their lifelong experimental studies. The ants use the sun as a compass, but a compass is not enough; the ants, when released from a point they had not themselves traveled to, like the bees caught in one pasture and released in another, apparently got lost.
For homing you must know where you are on “the map” before you head off in the correct direction. The desert ants can return home, but only if they walk to where they find themselves. Wehner concluded that the ants’ homing mechanism involves somehow calculating where they are at all times, probably in measuring distance by keeping a kind of count of their steps, and also keeping track of the angles of their direction from their home relative to the sun’s location. These were not mere speculations, but a hypothesis tested in painstaking experiments that entailed altering the ants’ perception of the sun (holding filters over them that varied the direction of polarized light that they, like bees, use in orientation) and altering their stride length (altering their leg length by gluing on extensions) to find out what information they valued and how they used it. Presumably bees could also have a similar “map sense,” and Randolf Menzel, a neurobiologist in Berlin, was trying to find out how it might work.
Menzel runs the large and active Institute of Neurobiology at the Free University of Berlin, and one of his projects was the burning question of how honeybees seem to find out where they are in order to be able to go where they want to be. Honeybees are suitable animals with which to study this problem because, like ants, you can count on their motivation to return home after they are loaded with food.
We can’t look into a bee’s brain and determine what it knows and what it wants. However, clever experiments based on the bee’s natural history permit inferences. We can determine, for instance, where a bee perceives herself to be relative to her hive. If a bee regularly visits a feeding place, she knows where she is, because she always flies off in a straight line from it back to her hive. If we then remove either the hive or the feeding spot, she circles in the area where her target had been. We know what she is looking for, because when we provide the hive and/or the feeding station within the area where she circles, she quickly finds it. But suppose we capture our bee at the usual feeding station after she tanks up on honey or syrup, put her into a dark box, and then carry her “blind” to a place she has never been. As mentioned, most bees will then make a beeline in the same direction they had normally flown to return to the hive. They will fly as far as before but find no hive there. Yet, they usually eventually do make it back home. How do they find their way? What do they do until they reach home? Until Menzel’s experiments, it had not been possible to track them in flight when they were out of sight out in the field. Menzel had a tool — radar but with a unique twist — whereby he could trace bees’ actual flight paths over a kilometer away by radar and record them on a computer. And he invited me to come see the work in progress.
Bee flight paths. A. A bee’s first trip from a flower patch or bee box back to its bee tree (hive) begins with an orientation flight. B. Later trips are more direct. C. After a bee has been transferred while “blindfolded” to a new spot, she acts as though she perceives herself to be still at the same place as before.
The problem of tracking small objects such as insects from a long distance by radar had always been that radar would “see” too much. You could not isolate and then plot a single specific bee out of all the extraneous noise of echoes bouncing off all objects. The new insect-tracking radar technique started in 1999, when Joe Riley, a British researcher, applied a radar system able to track very small objects over long distances by attaching to the insect a small device that, after receiving the energy of an electromagnetic sound pulse, would respond with a frequency other than that of the transmitted ultrasound. The receiver is then tuned to amplify only that frequency. In this way, it became possible to track the flight paths of individual preselected bees equipped with the appropriate transponders because the echoes from all other objects were filtered out.
The Menzel group’s electronics technician, Uwe Greggers, adopted the Riley system in 1999 and 2001 and got interesting results, but then ran into software problems. Nevertheless, given the promise from the data they did get, the scientists contacted a radar specialist at Emden (north Germany) who agreed to develop the system. The Menzel group then needed to find the right site in which to use it. They needed to locate the experiments at a large flat area devoid of trees in order to be able to record the complete flight paths without interference such as the bees’ trying to avoid objects or being attracted to them. The closest suitable area was an expanse of marshy meadow about a two-hour drive from Berlin. The large, idyllic farmstead near the village of Klein Lübben and land associated with it had accommodations for seven or more helpers, making this site amenable.
One Menzel group experiment in the works when I visited involved training individual bees to expect food at two widely separated feeding stations, but only one station at a time was open to them. I had no idea what to expect, and on my day with the team I was eager not only to watch the bees but also to see the experiment in action.
It was early in the morning when Menzel picked up Greggers and me for our trip to the experiment site in the Brandenburg countryside. We loaded a large, heavy printer that would be used to handle the large-scale printouts of flight paths, and then we were off down the Autobahn. Two hours later we arrived at Klein Lübben, a quiet village of farmsteads that at least in outward appearance has changed little since medieval times. The fields were several kilometers square, flat, and moist — perfect also for frogs, and hence storks which nest there in baskets attached to the tops of red-tiled house roofs. Swarms of starlings swirled through the air, and a pair of white swans paddled serenely down a canal along a dirt road, followed by a line of five still-downy gray cygnets.
At one end of the study field stood a steadily turning radar apparatus with a large round antenna for sending out the signal. A smaller dish antenna mounted directly above it would receive the transformed signal bouncing off the transponder on an airborne bee in the field. On the field sat two blue triangular tents and three yellow ones. They were experimental landmarks for bees that could be made available to them, to find out if they used them, and manipulated for experiments by changing their locations. In the distance sat a beehive, and I noticed a man running from it. He was wildly slapping himself, in an obviously defensive mode. He had been assigned to provide food for the bees close to the hive and then was to gradually move the feeder into the field so that a population of bees from that hive would be available for us to study when we arrived at midmorning. He had come too close to the hive, and at that moment it was he who was getting dispersed over the field, not the bees. Also, as we soon found out, there were no bees coming to the two feeder stations, as they were supposed to have been by now; the student had apparently overslept or been otherwise distracted from his assigned job of luring bees.
The experiment we wanted to do was in doubt. This was serious. Two hundred thousand Euros had already been spent on this study, and the boss was intolerant of negligence. Luckily, bees from hives used previously for another experiment were still coming into the field to search for feeders. He could let some of those bees find the feeders and then train them to come back to specific locations.
For our experiment we needed to establish two feeding stations, A and B, separated by about three hundred meters. Certain bees were already keyed into the routine. When I walked across the field, one bee started following me. It looked most extraordinary: it had a lot of blue and green color, not just the usual plain brown honeybee attire. As soon as Menzel’s helper and I set up our feeder, this specific bee landed on it and immediately started to suck up the rich sugar solution. Now I could examine her more closely: the green was a plastic tag with the number 29 on it that had been glued to her thorax. The blue was a slash of paint that had been daubed onto her abdomen.
Within a few minutes an assembly of several differently color-coded bees was lined up around the edge of the syrup dish. All were sucking up syrup. Some had green on the thorax, some had blue, and still others had yellow tags on their thoraxes, with additional daubs of white, blue, or yellow paint on their abdomens. Uwe Greggers and the unfortunate helper immediately started logging a list of the bees that had shown up in a notebook.
Each bee tanked up quickly, flew off directly toward her hive at the other end of the field, and then came right back to take a next load. Newly recruited (unmarked) individuals were also coming every minute to our site A. At the second feeder (site B) there was a similar flurry of activity, except it involved different individuals.
Menzel then instructed us to move our food station A one hundred meters closer to the second one, B. Bee numbers 29 and 30 green, both with blue tails, number 2 yellow with white tail, and number 39 green with red tail (who had all been present at A) then almost immediately started showing up at B, the new location. When crowds of bees had done the same, we removed one station and put the remaining one into the middle, between the two original sites. Next we moved our feeder to site B. Most of the bees, such as 30 green with blue and 39 green with red, who had been at the previous site, showed up. That is, we had trained bees who had been at one site to come to the second site, so we knew they now knew two sites and could potentially use either as a reference site to return home.
For the planned experiment it was important that the bees forget the intermediate sites that had been instrumental in getting them to go to the two widely separated sites. So, for the rest of the day, we alternately fed the bees, first at site A, then at site B, and monitored which individuals were showing up at both sites (most of the individuals continued to forage at either one or the other site).
We were now, near the end of the day, finally ready to move from training to trials. The experimental plan was to select one of the bees who knew both sites. This bee would, after feeding at one site and getting ready to leave, be captured in a dark box and thus “blindfolded” and then brought to a third feeding site where she had never been before. Here she would be released after being equipped with a radar-tracking transponder. We presumed that she would do at least one of three things: she might recognize where she was and fly straight home; she might instead fly off in her original (now wrong) direction; or she might immediately know that she was at an unknown location and search until she found one of her two feeding sites and from there take a direct beeline home. Knowing her exact flight path would allow us to distinguish among the alternatives, which would be essential to ultimately decoding her homing mechanism. Setting up this experiment had taken a long time, but I would now, possibly, be treated to an exciting demonstration of bee homing, one I could never have imagined possible.
Menzel picked up his walkie-talkie to call the radar station: “Mike, we’re now going to put a transponder on a bee — are you ready?” Mike had spent some years in the army where he was trained on radar, and he was now working part-time while getting a university degree in computer technology. He replied yes, he was ready. Menzel then took me to feeding station A, where a whole lineup of bees was coming and going.
“Which one do you want?” Menzel asked me. I wanted a bee that I had gotten to know over the course of the day, so I chose 39 green with red-tipped abdomen. We waited for her to arrive and let her feed for a while. As planned, Menzel then held a glass vial over her while she was distracted sucking up syrup. When she was full, she walked up into the vial, and Menzel corked it shut and darkened it by wrapping his hand around it. We then took her to a site distant from both feeders, a place where she had not previously fed and from where we would now release her.
The vial holding “39 green” had a plunger at the bottom with a wide-mesh screen at the top. Menzel gently pushed the plunger in and forced the bee up against the screen, held her there, and picked up a tiny transponder (a wire holding a diode with a sticky pad at one end). With fine tweezers, he deftly removed the protective paper from the sticky pad and glued the transponder onto the top of the bee’s thorax. “Ready?” he radioed Mike.
“OK.”
Menzel removed the plunger and held the vial with the open end up, for the bee to crawl up. She hesitated at the lip of the glass, groomed her antennae, and then lifted off. She showed no strain in flight. (The transponder’s weight is twenty milligrams, and a bee can fly with double her body weight, carrying a hundred-milligram load of nectar in her honey stomach plus two pollen packets on her hind legs.) However, she flew only two or three meters before dropping down into the grass, stopping to preen herself some more. But a couple of minutes later, she finally took off again. Mike, who was now monitoring her flight, radioed us. At intervals we heard: “She is heading south-north-east-north-west-south.” Then, finally, Mike continued: “Now her path is straightening out — now she is heading directly for her hive!”
She had suddenly oriented correctly. This was the crucial point: she had apparently recognized something that had “placed her on the map,” so that she then “knew” in what direction to fly to reach home. Assuming she had taken a path she had never taken before, did her successful homing suggest a “map sense”?
I ran over to the radar tent where Mike showed me the radar screen and the dots where the three-second successive readings traced the bee’s path. A computer screen, where software had converted the time and directions of the bee’s flight path into different-colored images for easy reading, showed that the bee’s original flight direction was toward where the hive would have been had we not moved her from her feeding spot. In other words, she acted as though she didn’t know where she was when we released her. As expected, however, after she reached the area where her hive would have been, she flew loops in several directions. Then, after she had flown ever-farther away from both her real and “would-have-been” hive locations, she suddenly seemed to orient and flew directly toward the hive. Amazingly, it was along a route that had not been her normal foraging route from her two feeding sites. Had she perhaps seen a blue or a yellow tent and, having learned their relationship to each other during previous orientation flights, transposed that information to fix her new location? Only more bees could tell.
Other bee homing experiments with hundreds of bees were ongoing. And in the group’s final publication two years later, the thirteen-author research team headed by Menzel concluded that honeybees incorporate information for flight direction from both their previously learned flights as well as landmarks and from the flight directions learned from hive mates within the hive. But they can redirect their flight vectors to and from the hive and perform novel shortcut flights between the learned and the communicated vectors.
“The” homing instinct, recognized and traded on by every American beeliner to get honey, and used by von Frisch to decipher the bee language, is a source of fascination and mystery still. Von Frisch had likened it to a “magic well” from which the more you take, the more runs back in. The “well” is still doing that, three-quarters of a century after his prophetic pronouncement.
GETTING TO A GOOD PLACE (#ulink_a1addad7-22a9-51d5-ad06-01f378599df8)
THE TENT CATERPILLAR MOTH, MALACOSOMA AMERICANUM, is common in North America. It emerges from its light yellow silk cocoon in late summer, and the female is then ready to deposit her batch of over a hundred eggs. She searches for an apple or a cherry tree, and somewhere out on a thin twig of just the right diameter — about a half centimeter — she exudes her eggs along with sticky foam to form her egg mass into a ring that wraps around the twig. The foam dries and hardens, encasing the clutch of eggs and gluing them solidly to the branch where they stay through the coming winter. But the larvae develop inside the eggs during the summer and, while confined in their eggs through the winter, hatch at almost precisely the day, about nine months after the egg-laying, when their tree breaks its buds.
The moth is named for the conspicuous communal homes of silk, called “tents,” that its caterpillars make, and in the spring of 2013 I found a just-made tent on a young black cherry tree next to my Maine cabin. Like nearly everyone else in this part of the country, I was long familiar with these caterpillars but had not deemed them worthy of a closer look. The tents act, I learned, like miniature greenhouses and warm the new caterpillars at a time when nightly frosts are still common. But, despite its advantages, to have any home is to incur costs: it has to be made, and it takes time, energy, and expertise to make, and the wherewithal to travel to and from it. For the time being, I wanted to know where the caterpillars making this home had come from. To my surprise, the ring on the twig with the now-emptied eggs I was looking for was almost a meter from the tent. How had the many hatchling caterpillars “decided” or been able to stay together and then coordinate to make their tent? Squinting against the sun, I could see a glistening trail of fine silk leading from the emptied egg-case ring to their home, so here was at least a hint as to how they crawl together to end up at the same place.
On the second day after I found the tent, May 1, there was still snow on the ground in the woods. There was as yet no sign of fresh green anywhere. But I wrote in my journal, “Black cherry buds ready to pop leaves.” These trees are the first to leaf out, and the caterpillars could not have fed yet. What would they do? An hour after the sun came up, the tiny caterpillars emerged from their tent and massed on its sunny side. An hour later they started milling about, and then a few started crawling, seemingly aimlessly, several centimeters up and down the trunk and branches of the cherry tree.
As I had anticipated, some of the tiny caterpillars started to crawl back onto the same branch they had come from, possibly following their previously made silk trail. But they went only six centimeters before turning back. Others went down the trunk of the tree. Always some would turn back, and then the others followed one behind the other in a line. Finally, by 7:30 a.m., a contingent of about twenty of them had progressed nine centimeters down the tree trunk, although two were coming back up. Then more started to leave the tent, and eventually all were in one long line, going only down the trunk and then angling up another branch. In half an hour the leaders had traveled seventy-three centimeters and reached a bud. The rest were strung out all the way to the tent, but their two other travel-direction options had been abandoned. All were eventually massed at the same cherry bud, three-quarters of a meter from their tent, and in an hour and a half they had all returned to their tent, one following the other in a long train.
The young black cherry tree showing relative locations of a tent caterpillar moth egg cluster (C) from which the clutch of just-hatched caterpillars emerged and traveled to start making their home (H) in a crotch of the tree, and their first travels as a group (T) to feeding places
At noon they came out and crawled onto the outside of their tent, waving their heads back and forth, apparently weaving silk from their salivary glands to enlarge it. Another hour later they were again all massed inside the tent and perched, immobile, tightly against the bark, where they were barely visible through the thin gossamer veil of silk.
The caterpillars stayed in their tent through the night, and I expected them to go at sunup to the same branch where they had been the day before. But instead, this time they all followed an entirely different path, going directly up the tree instead of down as on the previous day, and without taking another side branch. I could not detect any silk on their so-far two different foraging trails, and this time they went even farther — a distance of 130 centimeters. After their one meal the day before, they were already noticeably larger. A few were the same size as the day before, but most had probably doubled in weight. There were many tiny fecal droplets in their web. So they had fed, even though it seemed hardly possible that they had anything to feed on at the barely opening bud.
On the third day the buds had opened and the tree was replete with new small leaves pushing out of the buds. But it had been a cool night — there was again frost on the ground at dawn — and the caterpillars made a slow start.
The pattern soon became clear: the caterpillars spent most of the night and most of the day when they were not feeding in their home. The time spent on tree branches was brief, and it could not have been just to keep warm that they stayed in their home because they went back inside just as quickly after feeding regardless of temperature or time of day.
Having found and watched the caterpillars of one tent, I then observed others for more clues to their homing behavior. One of the surprises to me was that as they grew larger, they foraged independently of one another, no longer going to and from feeding areas in groups. Furthermore, after they were about half grown they left their tents, not to return at all but still to continue feeding before eventually searching for a spot in which to spin their flimsy cocoon. Tent caterpillars usually choose a bark crevice to pupate, although commonly they also choose the cracks in the sides of buildings. But why were the young caterpillars strongly homebound and the older ones not?
I suspect the young ones’ web-making behavior may have evolved in part as an anti-predator response. The tents were visited by red wood ants, Formica rufa, and right after the caterpillars hatched, these ants often loitered alongside them on their trails. I tore a nest open on one side to find out if it served as protection. It must have, because ants entered, though frequently wiping their antennae as though irritated. Nevertheless they tarried inside the damaged nest, and I saw them grab and walk off with caterpillars. No ants entered an intact nest of the several I watched, each of which consisted of several successive layers of silk. Thus, the webbing of the tent acts as a deterrent to predators such as ants. Staying inside the home most of the day and night, as these caterpillars appear to do when they are small, probably reduces mortality from parasitic flies and ichneumon wasps as well. When they are larger, the caterpillars are probably protected from the ants, as well as from most birds, by a layer of fine spines. They pupate without having to bury themselves to escape frost, because the adult emerges long before there is any frost.
Because these caterpillars are protected from predators in the summer homes they build and by the spines they wear, because they mature early enough in the summer for the pupae to avoid the cold of winter (by early emergence of the moth), and because the eggs and young larvae are immune to freezing because of the antifreeze they contain, “everything” in the life of the tent caterpillar moth may be found within a few meters. The adults that emerge in late June are not far from the apple or cherry trees where the parent left her eggs, and their life cycle can be completed without their having to go far from home, unlike some other insects which traverse a continent to be able to satisfy all their needs.
Monarchs. Of all the insects, the travels of the monarch butterfly, Danaus plexippus, are perhaps most famously spectacular in both scale and scope. Dr. Lincoln P. Brower of the University of Florida in Gainesville (now at Greenbriar College), who has studied this butterfly and its migration for over forty years, records the rich history of the emergence of our knowledge of monarch migrations. Early naturalists saw “immense swarms” in the prairie states where the caterpillars fed on the leaves of the many native species of milkweed (Asclepias) and the adults fed on the nectar of their flowers. Monarchs declined when later industrial agriculture destroyed many of their food plants, but in the nineteenth century they resurged in the East due to land clearing and the spread mainly of one milkweed, A. syriaca. Millions of them were seen passing for hours, even in Boston. This was a phenomenon that is hard to imagine now and it ignited much interest then. Charles Valentine Riley, the entomologist who first hypothesized that these butterflies engaged in a birdlike migration, cites people seeing them in the fall in swarms that extended for kilometers and obscured the sun, “blurring day into night.” Huge lines of them passing Boston in 1880 were described as “almost beyond belief.” Now, with reforestation, plowing, and then the use of Roundup and other weed killers that eliminated their food plants in agricultural fields, the monarch is but a shadow of what it was. In the past several years in the East, it seems to have almost disappeared. For the first time, I saw not a single one in late summer of 2013. But our knowledge of the scope of the monarch migration has blossomed.
Monarchs migrate on their own power for thousands of kilometers, and, unlike many other insect migrants, the population (though not the individuals) has a regular two-way migration, although as with the other insect migrants, the individuals that come back are not the same ones that left.
Unlike most of the other North American butterflies and moths, which overwinter in New England as eggs, larvae, pupae, or adults, monarchs cannot survive there through the winter in any stage. The population that normally now graces fields all along eastern North America overwinters at around three thousand meters’ elevation in dense fir groves on the southwest slopes of volcanic mountains thousands of kilometers to the southwest, near Mexico City. The monarchs find shelter in those fir stands from rain, hail, and occasional snow. It is not cold enough for the butterflies to freeze there, but it is cool enough for them to conserve the energy resources that they have accumulated on their way south.
The monarch butterfly adult, caterpillar, and chrysalis
In the summer, the monarchs fly in what look like random zigzag patterns over the New England fields as they stop here and there to sip nectar. Occasionally you see a mated pair, the female doing the work of flying, the male dangling passively with folded wings while attached by his genitals. After the prolonged mating (and/or technically “mate guarding,” since it prevents mating by other males), the female glues her delicately patterned green eggs with gold markings, one at a time, to the undersides of milkweed plants. In a few days, the flashy yellow-black-white larvae hatch and start chomping. After about fifteen days (depending on the temperature), the caterpillars have increased their weight to 1.5 grams (2,780 times the hatchling weight). The caterpillar attaches itself to a support such as the underside of a leaf by a clasping organ at the hind end of its abdomen to hang upside down. It will then molt into the bright green pupa (chrysalis) with the shiny golden spots that is surely familiar to almost all school kids. In a few days, the chrysalis starts to turn dark, and the outlines of the orange-patterned wings are visible through the now-transparent cuticle. When the chrysalis splits, along a predetermined line of weakness in the back, the limp adult slips out and expands its wings, and in two or three hours hormones will have instigated a biochemical process that hardens its body armor and stiffens its wings. The butterfly is ready to fly. Where will its wings take it?
Thanks to the monarch studies initiated in 1935 by Dr. Fred A. Urquhart and his wife, Norah Urquhart, from the Zoology Department of the University of Toronto and continued to the present day with the input and cooperation of thousands of amateur volunteers, there is now an amazing story to tell. The Urquharts noted in the late 1930s that the monarchs they saw in late May and early June in Canada had tattered wings, and they knew that this species would not and could not overwinter in Canada, so they suspected that they may have come a very long way. Monarchs fly in a southwesterly direction in the fall, but nobody had a clue where they ended up. To get some idea of the butterflies’ movements, these researchers in 1937 began gluing paper tags onto monarch wings with this message: “Please send to Zoology University Toronto Canada.” Monarchs weigh almost half a gram and the wing tags only 0.01 gram, so the tags were not likely to hamper the animals’ movements. Similar tags, used today, have pressure-adhesive backing and can be folded in half and glued over the leading edge of the forewing (after the scales are removed).
The idea from the inception of the monarch-marking studies was to try to find out if the butterflies migrated — an idea that at the time, as Urquhart noted, “was considered quite impossible.” But the question of where the butterflies might be going to and coming from grabbed the imagination, and anyone seeing a tagged butterfly would be sure to try to catch it. Sure enough, tags were returned over decades that suggested a migratory pattern. Individual tags were returned from huge distances, up to 1,288 kilometers. One monarch that was tagged in Ontario in 1957 was recovered eighteen days later in Atlanta, Georgia, 1,184 air kilometers distant. Clearly, when the butterflies left Canada in the fall, they headed south.
Still, nobody knew what happened to the mass of butterflies. Then, in January 1975, Cathy and Ken Brugger of Mexico City found them — a dazzling, shimmering, orange display of an estimated 22.5 million monarch butterflies on one 2.2-hectare site (which turned out to be only one of ultimately thirteen overwintering sites in the mountains of Mexico). The millions of monarchs were festooned in the trees in the mountains of Michoacán near Mexico City. The Urquharts excitedly traveled to see the site and on January 18, 1976, listened to “the sound of the fluttering of thousands of wings [that was] like that of a distant waterfall.” As they stood awestruck by this dazzling display, a pine branch broke off from the sheer weight of butterflies attached to it, and it crashed to the ground right in front of them. Fred Urquhart had been posing for a National Geographic photographer surrounded by these just-fallen butterflies when, incredibly, he saw a tagged one among them. When he traced its origin, he learned that it had been tagged on September 6, 1975, by Jim Gilbert, from Chaska, Minnesota. Urquhart, who had encountered countless tagged butterflies in his career, said it was “the most exciting one I have ever experienced.”
The picture that has now emerged from decades of study is that individual butterflies migrate all the way from Ontario to Mexico in the fall, arriving there at their overwintering sites in a torrent during October. They spend most of the winter in Mexico in a cooled low-energy state but soar around on warm days to drink water and replenish on nectar. In early spring, when their sex urge awakens, there is a mating orgy followed by a mass exodus. Most of the females mate before leaving, and their “compasses,” which were set to take them south in late fall, are now “reset” to take them in a northerly direction.
As the tide of butterflies advances northward, the females stop to lay their eggs on milkweed. Some of the butterflies from Mexico make it all the way to the north, and others (their offspring) that grow from the eggs laid along the way arrive later. Those of the first generation have slightly tattered wings when they arrive in the north, while those that arrive later have untattered wings. (However, not all monarch populations migrate, and not all that do, travel in the same directions as the populations of northeastern North America.)
One of the mysteries that puzzled Fred Urquhart was how the butterflies home. In Urquhart’s 1987 book on the monarch, he speculated that the butterflies perhaps use the Earth’s magnetic lines of force, although different populations of the butterfly migrate in different directions, so they could not all be orienting to it in the same way.
A potentially even more puzzling question is the ultimate (evolutionary) one of why these butterflies migrate in the first place. Urquhart simply suggested what he admitted was a “perhaps far-fetched” idea: that “twice each year it [Earth] passes through an area rich in some sort of radiation that could impinge upon animal life [that] might affect in some manner the cells of the body causing reproductive organs to abort in the fall and develop in the spring and initiate the migratory response.” This is an unlikely theory, though, mostly because it depends on a mechanism that is not adaptive in evolutionary terms. Instead, more current thinking about the adaptive reason why the phenomenon has evolved focuses on energy economy and maximization of resource use under the expected evolutionary constraints from the monarch’s having evolved in the tropics, meaning it was not able to survive northern winters. (Monarchs belong to the family Danaidae, an otherwise strictly tropical group.) Migration to the north in the spring opens up the milkweed crop over a major swath of North America as a food base for the larvae. In addition, the journey is probably not costly to the monarchs, either in terms of predation (since they are chemically protected from predation by poisons they sequester from their food plants) or in terms of energy costs, since their energy intake along the way more than makes up for the energy expended for travel. Indeed, unlike most birds that may deplete all their fat reserves on migration, these butterflies instead fatten up on their journey and may consist of about 50 percent body fat by the time they arrive in Mexico, where their overwintering fast begins.
Butterflies and moths experience tremendous selective pressure, and undoubtedly there are constant readjustments of survival strategies. Weather affects the populations, not only through flight activity and flight range as well as growth rates of larvae, but perhaps also indirectly by influencing virus infections. But Urquhart noted that each female monarch butterfly lays up to seven hundred eggs, and he calculated that the “biotic potential” — the number of individuals if there are no deaths — of one female after only four generations (that is, at the end of one summer) is 30,012,500,000 adults. Luckily for the planet, animals’ reproductive potentials are never naturally realized, for long. The limit is quickly reached when the population uses up its food base, in this case milkweed. In some years a virus decimates most of the monarch population over North America, but then several years later it rebounds. But the population cannot rebound from some things: in recent years there have been massive declines of the monarch population that cannot be reversed, because they are due to unnatural causes — the massive conversion of land to crops, and the introduction of genetically modified crops that tolerate herbicides, which have allowed the elimination of milkweed that formerly grew between rows of corn.
The flight performance of monarchs is spectacular, but like the hordes of cluster flies from the surrounding fields and woods that overwinter in my cabin, they are traveling to a specific place for overwintering where they have never been before. Such homing movements are diverse, but common. Robert D. Stevenson and William A. Haber of the University of Massachusetts, Boston, found a regular seasonal migration of about eighty percent (250 species) of butterflies living in the dry lowlands of the Pacific Slope of Costa Rica that migrate to wetter forests of the east. Distances traveled range from ten to a hundred kilometers.
In North America as well as in Europe, the cosmopolitan painted lady, Vanessa cardui, a mostly orange and black butterfly with white spots and pink and blue “eyes” on its under-wings, at times appears in large numbers and then is not seen again for years. Usually the individuals are seen crossing a road, and almost all will be heading in the same direction. The painted lady regularly migrates north from Mexico, from where it originates, after heavy rains in the deserts have created an abundance of food plants, primarily thistles. A friend told me of one migration while he was in Arizona when his windshield wipers “soon became useless” because of the huge numbers of painted ladies plastered onto them as he was driving. I see them regularly in Vermont and Maine, but seldom in large numbers (the summer of 2012 was one of the exceptions).
Red admiral butterfly larva, adult, and chrysalis. The larva makes a shelter for itself by pulling leaves together and holding them with silk, while then feeding on the leaf.
One of the butterflies that not only migrates as an adult but also hibernates in some parts of its range is the red admiral, Vanessa atalanta. It is (as are all butterflies!) beautifully colored. It sports a wide red stripe across each dark forewing ornamented with white spots, and its larvae feed on nettles. I wrote in my journal on May 11, 1985, near my home in Vermont: “In the afternoon from around 2:30 to 4:30 PM, as I was jogging along on an 18-mile circular loop I counted 512 red admiral, crossing the road in front of me. All but 5 of these were flying in a northeasterly direction. At 5:00 PM, after I was home, I take compass readings of butterflies flying over a plowed field where they funnel onto it through a valley. I can see them to take a bearing for at least 50 paces — 250 feet. All 22 that I observed flew in NE direction. At 6:00 PM activity almost stopped. The breeze is slight, from northwest.” In the summer of 2001 and again in the spring of 2010 I saw large numbers of red admirals. They fed on freshly opened apple blossoms, and later all the nettle plants in a neighbors’ sheep pasture had an abundance of their caterpillars.
Moth migrations are perhaps more spectacular than those of butterflies. Jason W. Chapman and colleagues report one recent ten-year study involving radar tracking of about one hundred thousand owlet (Noctuid) moths, primarily the silver Y moth, Autographa gamma, migrating south in the fall from northern Europe, and then north from the Mediterranean in the spring. Like the butterflies, these insects breed along their migration route. Also like the butterflies, the moths partially correct for crosswinds, to maintain specific directions. Most surprising perhaps is the moths’ windsurfing; they choose the most favorable wind currents corresponding to their respective spring or fall migratory directions. If the wind shifts about twenty degrees from the favorable direction, they adjust their flight to accommodate and maintain the correct direction. If the wind shifts ninety degrees, though, they stop and wait for a favorable wind. Millions of them fly together in the dark of night, and, like the monarchs’, their compass directions are likely tuned to the Earth’s magnetic fields. Some studies of radio-tagged green darner dragonflies, Anax junius, suggest that these insects also migrate hundreds to thousands of kilometers from north to south with those that return being a different generation.
These behaviors get the animals to a good place (for overwintering or for reproduction). Like the long-range movements with specific endpoints on the map, homing to a good place is not always easily distinguished from moving out of a bad place. The behavior is a mechanism with deep evolutionary roots. Indeed, insect wings (and metamorphosis) may themselves have been an original adaptation for dispersal, to colonize temporary pools, animal carcasses, or other temporary resources. The first individuals to reach the resource won the competition to use it and multiply there, and these were more likely to be the ones that flew, and flew far and wide, rather than those that walked at random.
Wings and metamorphosis have lesser value in constant conditions. Some insects are able to respond in real time to the changes in conditions they experience (especially crowding), in that when they don’t “need” to disperse they either don’t grow wings (some aphids) or the muscles to power the wings are broken down and the amino acids from the protein are used instead to make more eggs (in some Hemiptera bugs). Often there are discrete “dispersers” versus “non-dispersers” in any given insect population, and the percentage of each depends on the quality of the home habitat and hence the relative cost/benefit ratio of moving versus staying.
Dispersing to “anywhere but here” generally applies to nonmigratory species that have no encoded or learned directions to go to but may have innate instructions to move in more-or-less straight lines rather than potentially going in circles in order to achieve distance. In Africa, dung-ball-rolling scarab beetles race away from their often thousands of competitors at a dung pile at night by using the swath of stars of the Milky Way galaxy as a reference. Swarms of insects feeding at dung and carcasses also attract predators, and as soon as they finish feeding, many distance themselves from those predators. I’ve observed blowfly larvae at animal carcasses keeping to almost perfectly straight lines in their getaway at dawn, by steering directly toward the direction of the rising sun. Mass movements sometimes observed in some rodents, such as lemmings and gray squirrels (as in 1935 in New England) following a population explosion after a superabundance of food, may be another example of dispersal to get to a better place, though not necessarily a predetermined one.
On the other hand, “true” migrants are able to utilize ideal conditions in two places, provided they vary predictably. Arctic terns, Sterna paradisaea, breed throughout the Arctic, then fly to Antarctica to escape winter when food availability declines and to arrive in spring and food again, a round-trip distance of nearly seventy-one thousand kilometers. Gray whales, Eschrichtius robustus, also feed in the Arctic in the summer but then travel eight thousand kilometers along the coastline to Mexico to bear their calves in warm waters.
Dispersers are not neatly differentiated from migrants, although the first commonly rely on passive mechanisms as opposed to the migrants, which move to specific goals by their own powers of locomotion. There are all gradations in between. Each case is unique, and there are thousands. Let’s look at more ways of getting to a good place, as represented by eels, a grasshopper, aphids, and ladybird beetles.
Eels. There are many species of eels, but the American, Anguilla rostrata, and the European, A. anguilla, the species with which we are most familiar, live most of their lives in freshwater ponds and lakes. For reasons that are not clear, though, they do not reproduce in their home areas. To the contrary, both disperse (or “migrate”) thousands of kilometers on a one-way trip to spawn and then die in the mid-Atlantic.
Just as birds and some insects use air currents, eels use water currents to help them leave their lifetime homes. After eels leave their freshwater homes and head for the ocean to spawn and die, their larvae then drift in the ocean currents for years. But eels’ dispersal behavior is anything but passive. Eels fatten up to prepare to leave their home haunts in the bottoms of freshwater lakes and streams. Before they head for the sea, they absorb their digestive tracts and transform themselves by greatly enlarging their eyes and turning silvery on their ventral side. The latter transformation produces counter-shading that reduces their visibility to predators below them in the open ocean waters.
The eels’ dramatic changes in behavior, morphology, and physiology that enable them to switch from living in freshwater habitat to open ocean highlight the operation of strong selective pressures. But why do they leave their homes in freshwater ponds where they grew up and lived most of their lives? Their one-way, once-in-a-lifetime migrations to their Sargasso Sea spawning grounds can’t be to find a better feeding ground, or to escape competition. However, I suspect what the behavior accomplishes superbly is that the adults, which are predators, do not come in contact with and feed on their own young. Is it a mechanism that has evolved because it reduced predation on themselves?
There are over six thousand publications on eels, but the life cycle of these economically important food fish is still murky and has a long history of speculation. For centuries, nobody ever saw a baby eel, and even now, their spawning has not been witnessed. Aristotle presumed that eels grew from earthworms. The first young of eels, transparent leaflike forms, were found in the open Atlantic Ocean. Gradually, as more of these leaflike creatures were collected, it was noted that they varied in size, and that the smallest ones were found south of Bermuda in the Sargasso Sea, which was therefore presumed to be the site of their origin, that is, the eel spawning area.
The eel larvae, after hatching in the waters of the Gulf Stream, drift north. Like plankton, they move at the whim of the prevailing oceanic current. As they grow from about five to six centimeters in a year, they take on a more eel-like form but remain transparent. They are by then able to swim and, presumably by scent, find and swim up a river. Unlike salmon, however, these transparent “glass eels,” as they are known at this larval stage, can have no specific home stream scent to follow, because they have experienced only the scents of the ocean.
The female glass eels at this stage, in early spring, migrate up rivers and streams along the East Coast of America. After two months in a river they grow to about ten centimeters. Now known as “elvers,” they are no longer transparent, and they enter lakes and become eels. These female eels live in lakes for eight years or more, fattening up (the males stay in the saline estuaries). When they achieve the right amount of fat, these females become sexually mature. Each develops a clutch of three to six million eggs, and then one fall the gravid females start their journey downstream back to the ocean, to the Sargasso Sea to spawn. Since the males don’t live in fresh water, somewhere in the ocean the females then apparently meet the males for fertilization.
As the Gulf Stream continues north beyond North America, the larvae of the European eel, which originate from the same apparent spawning area, the Sargasso Sea, as the American eels, continue their journey. Finally, in two or three years, they reach the coasts of Europe, where they then also seek rivers and streams. Migrating upstream, they become pigmented, and after growing to adulthood, they migrate back into the open Atlantic and make their sixty-five-hundred-kilometer return to the Sargasso Sea, where they spawn on average several million eggs, and then die. Only one of several million of them will make the return journey to grow to a reproducing adult in fresh water.
Grasshoppers. One of the best-known insect dispersers, the “migratory locust” (the grasshopper, Schistocerca gregaria), engages in some of the most spectacular mass movements in the animal kingdom. On the African continent, this species has been famous since biblical times. Swarms of the “locust” have blackened the skies, and as those in the vanguard settle onto the earth and consume every green thing where they land, the rest fly over them until they reach more green, while those behind then take flight and do the same, and so a horde of hundreds of millions moves along, stripping all vegetation in its path. Predators cannot put a dent in those hordes. Additionally, the migratory locust is distasteful to potential predators because when it migrates, it is not choosy about what it ingests and takes up toxins from poisonous plants, which it incorporates into its tissues. The grasshoppers’ bright red-orange and yellow coloration, like that of many insects including the monarch butterfly, reminds potential predators of its distastefulness.
Nymphs and adults of the two phases of the migratory grasshopper
Although this distinctively colored grasshopper appears to arrive suddenly, it is often there all along, but in a different guise. It has a green solitary grass-fed form that blends in with its food and that is palatable to predators. For a long time scientists thought that the grasshopper “migrants” that appeared so suddenly were a unique species, one arriving from an unknown origin and heading for an unknown destination. Now we know that the migrants are a “phase” of a common species that changes its color, form, and behavior in response to crowding. Proof comes from experiments: to create these “migrants” from isolated individuals one takes a nymph (immature stage), puts it in a jar, and has a motor-driven brush tickle it continuously. The constant tickling mimics the crowding, which in the case of S. gregaria is the signal evolution has “chosen” to trigger the nervous system to alter the hormones that result in development into the restless migratory phase of a different color, wing length, and behavior. It is a good example that shows environment is “everything,” or from another perspective, it’s all about genetics.
Migratory-phase locusts are highly irritable and will jump up and follow a crowd flying over it. This behavior removes the grasshoppers from an area that is overpopulated and brings them to new land where conditions are conducive to feeding, egg-laying, and growth of their offspring. Although the grasshoppers could have no knowledge of where such a distant but good place might be, they migrate to it as if they do.
The grasshoppers reach a consensus. It is a sensible one, although it involves no thinking and no discussion. They simply fly up to join the crowd, which follows the prevailing winds. Eventually these winds meet air from an opposite direction and, when moist tropical air rises into cooler altitudes, rain precipitates out of the resulting clouds, depositing the grasshoppers to earth along with the rain. As the ground is watered and softened, the grasshoppers can shove their abdomens into the soil to lay their eggs. The new nymphs hatch just as new food starts to sprout. Their homing (or “dispersal”?), which has ended at this good place for them to reproduce, is now complete.
Aphids live in crowded “colonies” on plants into which they insert their mouthparts, much as mosquitoes puncture skin, except that they imbibe plant sap instead of blood and may stay plugged in at the same spot for most of their life spans. One might suppose they could not or would not migrate. But, like the migratory grasshoppers, they may travel possibly hundreds of kilometers. Nobody knows for sure how far; it depends on the prevailing winds.
Sedentary aphids already on good food do not leave to seek, or even require, mates. Instead, they switch to virgin births after a sexual migratory phase. Daughters then settle directly next to mother, and so on and on for many generations as the colony grows. And then, cued by the shortening of the days in the late summer and fall when the food supply runs out, the aphids’ offspring take a different developmental route. Because of changing day length and/or food, the nymphs on their final molt grow wings and become sexual. Frail and weak-winged they are, but an aphid is light and carried by the wind much like the seed of a dandelion or poplar tree, or a baby spider on a thread of silk. I usually see them in September when they appear like flecks of white lint floating erratically in the air.
To reach wind the aphids fly or are wafted up. Eventually, they don’t fight the wind but drift along and settle somewhere back down to earth. On their descent, assisted by their wings, they head toward anything colored light green. This color (unless they are tricked by pieces of green paper coated with sticky glue left by an insect physiologist studying them) is likely to be associated with their favorite food, fresh plant growth. After landing, perhaps because the chances of a mate arriving at precisely this one tiny spot of residence are remote, they switch back to virgin births and thus restart the cycle.
Ladybird beetles, the predators of aphids, similarly have adapted by migrating in a seasonal environment. In the western United States they migrate mainly on their own power from lowlands up into the Sierras, where they can in some locations be scooped up by the bucketful (generally to be sold to farmers and gardeners — to control aphids!). At the campus of the University of California at Berkeley, I often saw streams of them flying or being blown uphill in Strawberry Canyon by the campus when the grass was drying after the spring rains.
Ladybird beetles of some species migrate when reproduction must cease for the season. Despite the energy they expend for flight, they may migrate largely to save energy. It goes this way: As long as there are plenty of aphids to be had, both larvae and adult ladybirds don’t go hungry. Eventually, however, the green vegetation suitable for aphids disappears in the hot California summer, and so the aphids leave. Now the beetles’ resting metabolism kicks in as a significant liability. Resting metabolism for beetles is high at high body temperature but becomes almost negligible when they are torpid at the lowest body temperature tolerated, near or slightly below freezing. An elevated resting metabolism, month after month in the western states’ dry hot summer, would deplete both the beetles’ energy reserves and their body water. Without replenishment of food and water in that environment they would die. But by flying, with the aid of thermals, they are brought into upwelling air currents in the hills and then into the mountains where they reach cooler air. At this point, though, they do something different from the aphids: instead of being attracted to green, they are attracted to either red and/or the scent of each other. How else to explain that they crowd together into large groups in which they then overwinter? The advantage of their grouping behavior is not clear, but I suspect that it amplifies their noxiousness. (This is based on experience: ladybird beetles regularly come into my cabin to overwinter and quite often crawl into bed with me. I can vouch for the fact that they are noxious if not obnoxious, and several more so than one.)
The ladybird beetles arrive at a suitable place — a cool one — where they conserve their limited energy reserves during winter. The hypothesis that they home not just to an area, but also to a specific spot, is based on observations that my friend and colleague Dr. Timothy Otter has made in the Sawtooth Mountains near Stanley in Idaho. The beetles there were known by local ranchers to aggregate every fall in large numbers in specific rock cairns of decomposing granite in the hills above the valley floor. Otter, a biologist, wondered why the beetles aggregated there, in specific spots, and not in similar places nearby.
Since hibernation concerns adaptations related to temperature, Otter concentrated his efforts on unraveling the beetles’ temperature tolerance and compared it to the temperature at that site. But, curiously, there was nothing unique about the temperature of the specific site on “Ladybug Hill,” relative to other sites near it. Nor did the grouping of the beetles affect their temperature; the temperature of their aggregation was indistinguishable from ambient temperature. So, they didn’t aggregate to keep warmer than the environment there.
As already mentioned, the one thing all ladybug beetles do have in common is that they stink. The evolutionary significance of their aggregations is therefore likely the conventional one of other animals, namely, to advertise their noxiousness for protection from predators. Shrews and other predators may kill two or three victims and then spit them out, but they then learn to avoid them and so they do not continue eating the hundreds of thousands they would consume if they had found a bonanza of ladybirds. Given their safety of numbers, it is advantageous for any one beetle to join a group rather than to overwinter alone, because the chances that it will become a victim of a shrew’s educational process are reduced in more or less direct proportion to the group number. This rationale for large numbers of beetles, but of different individuals, massing repeatedly at the same location is not proven, but my colleague Daniel F. Vogt and I found that it applied to another noxious-smelling beetle, the whirligig (Gyrinidae) water beetles in Lake Itasca in Minnesota. These beetles there homed in on any existing groups of tens of thousands at dawn, after a night of foraging far and wide on the surface of the lake.
The second question is: How is an aggregation formed?
Insects have an impressive ability to home in on scent, and ladybird beetles could find the aggregation by following an odor plume. Memory cannot be involved in the ladybugs that Otter found year after year aggregating at the same site, which were generations removed from those of the previous year. The beetles arrive on site in September, stay there eight months, and in May return to the valley floor to feed, mate, and reproduce. Only their descendants, two or three generations later, could return to the tiny spot where they had hibernated.
The aphids’ rule of flying up toward the light, to then be dispersed by the wind, and then homing in on green when they settle could be a model of what happens in ladybird aggregation. Ladybirds are much stronger fliers than aphids, although they too are swept along in updrafts. But such drifting, while helping to account for their annual ascent from the lowlands into the mountains, does nothing to explain how tens of thousands of them end up under the same rock pile.
If the ladybirds home in on color, this could be tested, as the aphids’ homing in on food plants was tested — by leaving color targets at sites other than the traditional hibernaculum. But even if red color is an attractant (highly unlikely because the beetles aggregate under the rocks, not on them), that still would not explain their annual return to the same place in successive years. Do they smell their ancestors? Could thousands of smelly beetles piled up for eight months leave sufficient scent residue to serve as a marker that allows others to home in on the spot? If so, this idea could also be tested, by transferring an aggregation of beetles to overwinter at another physically similar place in the same general area as the old to see if a new traditional homing site for their mutual protection is created.
BY THE SUN, STARS, AND MAGNETIC COMPASS (#ulink_476b060f-b928-5d0d-9e35-7a5aea75144f)
Life has unfathomable secrets. Human knowledge will be erased from the world’s archives before we possess the last word that a gnat has to say to us.
— Jean-Henri Fabre
CHARLES DARWIN REFERRED TO THE ACCOUNT OF FERDINAND von Wrangel’s Arctic explorations, The Expedition to North Siberia, concerning how we home, quoting von Wrangel on how the Siberians oriented by “a sort of ‘dead reckoning’ which is chiefly affected by eyesight, but partly, perhaps, by the sense of muscular movement, in the same manner that a man with his eyes closed can proceed (and some men much better than others) for a short distance in a nearly straight line, or turn at right angles and back again.” Darwin compared a bird’s homing capability with that of people, but much less favorably, by telling how John James Audubon kept a wild pinioned goose in confinement, which at migration time became “extremely restless, like all other migratory birds under such circumstances; and at last it escaped. The poor creature immediately began its long journey on foot, but its sense of direction seemed to have been perverted, for instead of traveling due southward, it proceeded in exactly the wrong direction, due northward.” I’m not the least surprised at the behavior of the goose but all the more puzzled by our own orienting, which involves knowledge versus a feeling of “sense of direction.” I recall instances of waking up in “total” dark, “knowing” in my mind precisely how I am oriented relative to the room and hence the rest of my environment, but irked by “feeling” that I am in the precise opposite direction. It is then a struggle to get the two to agree, which happens only after some effort.
In Darwin’s time it was still supposed that humans had overall superiority over other animals. His then-hypothesis (later theory, and now fact) of evolution, which now binds us all as kin, was still revolutionary. Darwin found the goose’s behavior puzzling because he could not know that geese, cranes, and swans stay together in family and larger groups and that although the young by themselves do not know the correct migration route, they learn to know it from their parents which in turn learned it from theirs. Other birds have their migratory directions genetically coded, and they go strictly by “feeling,” since many of these have no knowledge because they migrate ahead of their parents.
We humans get lost easily. We would not get far without reference to landmarks, and I base that conjecture on (inadvertent) experiments. In one I was in long-familiar woods and got caught in a heavy snowstorm. Suddenly I got “turned around,” and it seemed as if all landmarks had in almost one instant been erased. But I kept going, trying to maintain a straight line by trusting my “sense of direction.” When I thought I had reached a place that I knew, where I should be going downhill, the landscape was instead sloping upward, and the brook I had expected to see was going in the “wrong” direction. At that point, knowing I was lost and no longer referencing to any signal, I backtracked in the snow and discovered that I had been walking in a circle, all the while thinking I was heading in the “right” direction. Yes, we can walk, a short way, in a relatively straight line with our eyes closed, by a process dubbed “path integration,” but my emphasis here is on “relatively.” Mice may do better. A friend told me of catching a Mexican jumping mouse in a live trap baited with peanuts. It had a kink in its tail, so he called it Crooked Tail, or CT for short. After he had released it several times, and it always returned for another snack of peanuts at the same source, he finally decided to “test its mettle” and released it exactly one kilometer away in thick brush and grass. The next morning CT was back for another snack. After release from two kilometers, though, it did not return. We don’t know, though, if this was due to failed navigation, finding a new food, a cost/benefit calculation, or a run-in with a coyote, owl, or weasel. On the other hand, when I failed to navigate, I was positive that I had been going in the precise opposite direction, which meant I had no sense of direction whatsoever, except that coming from visual landmarks from which I had constructed a map in my head.
When we do home, it is by maintaining a constantly updated calculation from at least two reference points, and the motivation to use them. We are innate homebodies, normally seldom displaced, so that in our evolutionary history there has been little need for a highly developed home-orienting mechanism. Simply paying attention to familiar landmarks suffices. Males on average may perform better than females in negotiating unknown territory, and it is posited that they, having been hunters traditionally, have a better “sense of direction.” But I doubt it. Learning, and especially attention, is hugely important for a presumed directional sense that can be developed to a high degree, as shown in some Polynesian seafarers living on isolated islands. But basically that involves being alert to more cues. These seafarers had been trained from near infancy to “read” the stars, the ocean waves, the winds, and other signs so that they may navigate over vast stretches of open ocean. But what a select few human navigators can accomplish with experience and with tools, many insects and birds do routinely as a matter of course, and with far greater precision over distances that span the globe.
Every fall and spring billions of birds travel to their wintering grounds where they can find food, and in the spring they return to near where they were born in order to nest. In huge tides, partially aided by favorable winds but mostly by their own muscle power, they ply the skies in the day and at night in the Northern and Southern hemispheres, sometimes covering thousands of kilometers in a few days. For the most part, the birds have pinpoint home destinations, places such as a specific woodlot, field, or hedge. In the fall they reverse their journey, though often by a different route, again to reach specific pinpoints in their winter homes. Turtles on the seas accomplish the same navigation feats between breeding and feeding areas.
The magnitude of birds’ migratory performances staggers our imagination, in terms of both physical exertion and feats of navigation, because they are vastly superior to anything we could, as individuals, accomplish. Bird migration, as we now understand it, for centuries seemed impossible because we used ourselves as the standard, and that of turtles was not even considered. The animals’ performances would still seem impossible, given our ignorance and arrogance, were it not for the proof from countless research experiments.
The homing behavior of birds was known and used as early as 218 BC, when Roman foot soldiers captured swallows nesting at military headquarters and took them with them on their campaigns. They put threads on the swallows’ legs with various numbers of knots to specify perhaps some prearranged signal or information, so that the marked bird when released and then recaptured at its home nest would bring the message. Today, between 1.1 and 1.2 million birds are banded annually in America alone, providing an ever more detailed picture of where the different species travel and when.
As with insect dispersals/migrations, our attention and insights into bird homing were and still are stimulated by spectacular examples. We are perhaps most impressed, if not baffled, not only by the birds’ wondrous physical capacities, but also by the cognitive or mental capacities that underlie them. Seafaring animals, like albatrosses and shearwaters and sea turtles, are especially noteworthy to us because we can’t explain their behavior by the use of at least to us visible landmarks, our main if not only recourse.
The Manx shearwater, Puffinus puffinus, navigating over the vast oceans, was one of the first birds to excite our curiosity enough to spark examining the wonder of bird homing. Shearwaters never cross land. All their food is taken from the water surface. As with most birds, their young are fixed to a specific safe or sheltered place, in this case an island, where one parent may spend as much as twelve days at a time ceaselessly incubating before being relieved by its mate. They nest in a burrow in the ground on islands in the North Atlantic, making it quite easy to catch, mark, and release them to identify individuals. We can also assume that as with bees, their motivation is to return home, and thus they are ideal subjects for homing experiments.
Prior to the First World War, the English ornithologists G.V.T. Matthews and R. M. Lockley took two shearwaters from their nest burrows on the island of Skokholm off the southwest coast of Wales and released them from points unknown to the birds. Under sunny conditions, the shearwaters returned to their nests by flying directly in their homeward direction. In one such test, a shearwater was carried by aircraft to Venice — a huge distance from its nest and an area where no shearwaters occur. The released sea bird might have been expected to fly south to the sea. Instead, it headed directly northwest to the Italian Alps and in the home direction toward Wales, in a path it never would have flown before. It returned to its home burrow on Skokholm 341 hours and 10 minutes later. This could, of course, not have been a direct nonstop flight. Unfortunately at that time there was no way of knowing if it had stopped to forage and/or what route it had taken.
The experiment was repeated involving even greater distances, after transatlantic plane travel became routine. Two banded Manx shearwaters also taken from Skokholm were carried by train to London in a closed box and flown to Boston, Massachusetts, on a commercial TWA flight. This is perhaps the ultimate in terms of the “blindfolded” displacement that I previously described for experiments with honeybees. One of the birds did not survive the journey to America, but the other, which was released near a pier on Boston Harbor, “abruptly turned eastward over the ocean.” Dr. Matthews, a leader in the study of bird homing at the time who had released 338 Manx shearwaters on the British mainland, discovered the bird back in its home burrow before dawn on June 16, twelve days and twelve hours after it had left Boston, almost five thousand kilometers away. On reading its tag, Matthews sent a telegram to the person who had released the bird: “No. Ax6587 back 0130 BST 16th stop-FANTASTIC-MATTHEWS.” Making another round that night to check on the bird again, Matthews, as though not believing his eyes the first time, wrote in a letter (to a friend, Rosario Mazzeo) that he was “completely flabbergasted” and had to read the ring several times before putting the bird back into its burrow.
By 1994 biologists had attached radio transmitters to animals that sent out high-frequency radio pulses received by satellites orbiting up to four thousand kilometers away. When two satellites picked up the same signal, scientists could calculate the transmitter location and relay it to receiving/interpreting sites on the ground. There, computers tracked the birds’ positions and drew maps of their travel routes over months. From these and other studies, we have learned that these seafarers, and sea crossers, both turtles and birds, may wander over thousands of kilometers of the ocean vastness and then return to tiny isolated targets, the homes where they were born. They can travel in straight lines even at night and while correcting for the drift of currents or wind. Using the new technology, these behaviors have been demonstrated perhaps surprisingly in a sandpiper, the bar-tailed godwit, Limosa lapponica baueri.
The bar-tailed godwit, a shorebird that nests on the Arctic tundra, winters in the far south of Australia. It has a long thin bill for extracting worms from deep soft mud. This species makes its Arctic home on a shrubby hillside with low tundra vegetation and nests there on almost any of millions of hummocks to be found on the tundra in Alaska or Siberia. Its nest is a slight depression lined with grass and lichens. The female lays her clutch of four large olive-brown mottled eggs into it, and the pair take turns incubating for about a month until the fluffy young, in camouflage down, are hatched. The parents then lead their chicks around and they feed themselves.
The bar-tailed godwit is not a particularly unique shorebird, as such. (The Hudsonian godwit, Limosa haemastica, performs similar flights from Manitoba to Tierra del Fuego and back.) But in the past ten years, possible extremes of homing ability and some astounding physical capacities that back up this behavior have been revealed by Robert Gill Jr., a biologist with the U.S. Geological Survey, who deployed twenty-three godwits with either solar-powered backpack transmitters or battery-powered surgically implanted ones in the abdominal cavity. The transmitters trailed thin antennas behind the birds, and the radio signals from them indicated their location and were received by polar-orbiting satellites. The data of the godwits’ locations throughout their flights was then calculated on the ground. Nine of the transmitters functioned for two years, yielding data on both the southern fall migration to Australia as well as the spring migration back home to the breeding grounds in Alaska.
Flock of bar-tailed godwits on migration
They revealed the hugely surprising fact that the godwits make the flight from Alaska to Australia nonstop.
The godwits fly directly across the Pacific Ocean in six to nine days. One female covered 11,680 kilometers in 8.1 days in her southward migration, and another traveled 9,621 kilometers before she lost her transmitter after 6.5 days. When the birds arrive back in New Zealand or Australia after their transoceanic flight — with no feeding, no drinking, and presumably no sleep — they have halved their starting body weight.
Portrait of a bar-tailed godwit
The godwits’ northward journey to the breeding grounds may involve a different route, and this one includes stopovers on the way. These stopovers permit the birds to replenish so they don’t arrive emaciated just when they begin the most energy-demanding part of their breeding cycle. For example, one godwit, identified as “E7” (which covered twenty-nine thousand kilometers in a round trip from New Zealand to its nesting area in Alaska), on its northward journey stopped at several staging (refueling) sites in the western Pacific and Japan, from where it then made the relatively short jump to its western Alaska home. On the other hand, on its southern migration after the nesting, it flew directly south from Alaska across the Pacific and back to New Zealand.
Right after a male godwit arrives back at its patch of tundra that is its home in Alaska, he circles for hours high in the sky and calls loudly near this chosen home site. In as little as a week before, he may have been on a coastal mudflat in Japan, where he had a raging appetite and gobbled worms and crabs day and night. Similarly, to prepare for his departure before the Alaska winter freeze-up in the fall, he will feed until he has doubled and even almost tripled his body weight in fat. And then, by our standards, in grossly obese condition, he lifts off to fly south. Although some godwits will stop off briefly in the Solomon Islands and New Guinea, others will fly up to fifteen hundred kilometers per day without a single stop. On their stupendous flight the godwits use up not only their body fat but also protein derived from shrinking muscles and organs, including almost every part of the body except the brain. The flight muscles are the primary powerhouse for the effort, but the brain — the organ that drives birds’ motivation to keep going — is more important.
Why do the birds leave at all, or go so far? Why do they face the privations, risks, and exertion of the journey? What drives their rapid fattening up without which they could not have enough fuel to reach their distant destination? Only raging appetite would fuel the fattening. Only an unquenchable drive to fly would make them go and keep going. The motivations and the behaviors presumably evolved because the Arctic summer provides more food than farther south, and so many species became adapted to be at home in that habitat. On the other hand, the Arctic provides little sustenance for most in the winter. The great migrations were shaped, then, by these imperatives.
I may be anthropomorphizing to suggest the godwits have a “love” of home, but although we can never know what they feel, it is hard to deny that they do feel. We can say that, along with the aforementioned cranes Millie and Roy, it is highly unlikely that conscious logic could drive them from one continent to the next. Animal behavior is first of all driven by emotion, although in us the emotion can be secondarily buttressed and/or amplified by logic. That said, we admire emotions that help us accomplish great things. We admire the drive and commitment that the birds show because our individual extraordinary feats pale in comparison to those of a godwit. The first lizards that sprouted feathers on their forelimbs could shield themselves from the rain and cold and may have been able to glide several meters, but for that they probably did not need drive related to homing. To fly nonstop for eleven thousand kilometers over open ocean, though, without taking a bite of food, a swallow of water, or a minute of sleep, is a mind-boggling demonstration of the epic importance of home, and of the ability and drive to return to it of even tiny birds.
Consider the example of a common European garden warbler, Sylvia borin. It is born in May somewhere on the northern European continent. It never in its life receives any instruction on when and where to fly. But two to three months after its birth it begins its flight in the night to Africa, where it has never been before. After reaching the Middle East, having flown in a generally southeastern direction, it shifts into a direct southerly direction and crosses the Sahara Desert. It eventually ends up somewhere in a patch of thorn scrub in perhaps Kenya or Tanzania, where it remains until spring. It then returns not just to the north, but perhaps to the same hedge in Russia or Germany from where it came, and after nesting there it again flies south to Africa to the same patch of thorn scrub where it wintered before.
Songbirds in North America do much the same. The Bicknell thrush, Catharus bicknelli, lives in the summer in the spruce forests on mountains not only directly adjacent to my home, but throughout the mountains of New England, the Catskills, and eastern Canada. It spends winters in the cloud and rainforests of Jamaica, Cuba, Dominican Republic, Haiti, and Puerto Rico. Christopher Rimmer and Kent McFarland and colleagues have been tracking these endangered birds in both habitats, to determine their home requirements. McFarland is the associate director of the Vermont Center for Ecostudies and has banded nestling Bicknell thrushes in Vermont. The birds return annually to their same homes, and his first encounter of an overwintering thrush in the Dominican Republic turned out to be one he had banded nineteen months earlier in Vermont. He told me that capturing the same bird seemed like “winning the lottery while at the same time being struck by lightning. But for us naturalist types, much more exciting.” On this occasion he broke out the celebratory Dominican rum on the very first night of that trip rather than toward the end of the fieldwork, as is more typical.
Routes of long-distance homing are now well known, but the how of the travel and the orientations to specific points of destination are still tantalizingly far on the horizon. The how is the most challenging of all to comprehend fully, because it literally involves everything about the animal at once — senses, metabolism, emotions, mechanics — all the physiology that runs the brain and the rest of the body. Solving such problems requires access and repeatability; animals don’t migrate in the lab at one’s convenience. Only one piece, or a few interrelated pieces, of the puzzle can be profitably examined at a time. Usually one animal species, by some quirk of its biology, provides access to a specific piece of information and another provides an opportunity for access to another.
The common rock dove or “pigeon,” Columba livia, with its long association with humans, has provided clues to many aspects of homing. The same or similar general homing mechanisms of this “homing pigeon” could presumably also be used by migrant birds, and nonmigrating but far-ranging sea birds and turtles. Pigeons were well known since the Assyrians and Genghis Khan, who used them in war. Julius Caesar used them to send messages home from Gaul. They were used in the two World Wars and in the Korean War. Because of their attachment to home, they were ideally suited, as were swallows, for carrying messages, especially in wartime, as they were difficult to intercept and were probably more reliable for transmitting secret messages than the telephone and Internet are today. Fifteen-hundred-kilometer flights for birds in the U.S. Signal Corps are considered routine, and flights of twice that distance are recorded. One could release pigeons at any location and at any time and be assured they would try to return home, provided they were not too young.
One of the common sights wherever pigeons are kept is groups of them circling near their lofts in apparently aimless flight. Pigeons engaging in these flights are said to be “ranging” — they may be out of sight of the home area for a half-hour to an hour and a half. As in honeybees starting their foraging career, these flights are especially important for the young birds because during them they familiarize themselves with their home area.
Are the pigeons, like bees, using landmarks for homing? To test for this possibility, Klaus Schmidt-Koenig and Charles Walcott, both renowned bird orientation experts, put frosted contact lenses on pigeons’ eyes to prevent them from seeing landmarks. To everyone’s surprise, some of the pigeons, after being displaced, still managed to return to their home lofts. They flew in at high elevation and then fluttered down close to their home. The birds had apparently gathered some clues other than landmarks visible to us.
Through time and experience, and longer and longer ranging excursions, pigeons enlarge the area where they are at home. A working hypothesis is that “lazy” fliers, those that make only short flights, are unlikely to be able to home from long distances. Pigeon racers, who compete in the homing ability of their birds, bank on the knowledge that the longer the ranging flights, the swifter and the more accurate the homing ability. After about two weeks of ranging, the pigeon racer usually takes his or her pigeons farther away for each “training toss.” Typically, the first training tosses are about thirty kilometers from the home loft. After three weeks the distance is increased to sixty kilometers, and then after another week to ninety kilometers. The birds’ capacity gradually to increase their homing ability reinforces the notion that they are learning something about their home area, perhaps something like a “map” using some kind of landmark. Precisely what the birds are sensing at any one time that allows them to orient correctly to return home is not known, in part because it probably varies depending on the place and the situation. Although it is still not clear exactly how pigeons are able to home, we know that several senses are involved.
We have seen that some migrant birds stay together in family groups (geese, swans, and cranes), and that the migratory directions are learned from the parents, which expose the young to the relevant cues much like pigeon fanciers expose their charges with “training tosses” far from the home loft. The phenomenon of parental leading has been documented in whooping cranes, Canada geese, and ibises and extended by humans leading young tame birds to become imprinted on ultralight aircraft, in order to establish new migration routes. In most migrant birds, though, the migratory directions are inscribed in a genetically fixed “program.” In either case, the migrants travel between one fixed territory in their summer home, and another in their winter home. However, presumably other, especially complicated mechanisms of homing are required in sea birds, which range far over the oceans and sometimes return to only a tiny speck, their natal island, after having wandered from it five or six years before. Do they build a map in their brain of some features of the ocean terrain that we can’t see? In other words, do they see the ocean not as a flat, uniform expanse as we do, but instead as a featured pattern as of hills, valleys, ridges, and mountains in perhaps magnetic anomalies that inform them where they are at all times?
The one thing we now know for sure is that, like us and like bees, birds use the sun as a compass for homing. Gustav Kramer, a German ornithologist, perhaps the principal pioneer in homing behavior in birds, in the late 1940s tested the “sun compass” of pigeons in circular cages with food cups placed regularly around the periphery. The birds were trained to expect food in specific cups (directions). After the pigeons were trained, rotating the cage did not alter the direction where they sought food — except when the sky was overcast and the sun not visible, when they searched randomly for food at the different cups. Kramer repeated similar experiments with a well-known bird, the northern European starling, Sturnus vulgaris.
European starlings in Europe migrate south in the fall (though many or most of those now in Vermont and Maine do not), at which time they, as well as other migrants, enter a state of restlessness. Kramer coined the word Zugunruhe, meaning literally “migratory restlessness,” to describe it. He first noted this behavior in his caged starlings, which were agitated and hopping around in their cages in the spring and tended to orient northeast. They oriented in the correct migratory direction when the sun was out, but as soon as the sky was clouded they no longer oriented in any one direction. Suspecting that, like the pigeons, they might use the sun to orient by, he tested his hypothesis by showing them the sun in a mirror and found that they then reoriented to the reflected sun. But the sun moves through an arc from east to west throughout the day, so how can the birds keep a constant migratory direction? Was the starlings’ behavior a laboratory artifact?
In order to find out if starlings indeed adjust the angle of flight to the sun throughout the day, Kramer put his migratory restless birds into a room where they did not have access to sunlight. Instead, he provided a stationary light bulb to stand in for the sun. As predicted, if they used the light bulb as a substitute for the sun and possessed a time-compensated sun compass, the birds oriented increasingly to the left throughout the day. That is, they changed their intended flight direction with respect to the constant light bulb direction, treating it as though it were moving on the same schedule, of fifteen degrees per hour, as the sun does, and so they almost always faced in the “wrong” migratory direction in reference to the ground.
Kramer later lost his life while climbing a cliff trying to get baby pigeons to raise them for further experiments on homing orientation. But one of his students, Klaus Hoffmann, carried on his work. Hoffmann, who later worked at the Max Planck Institute for Behavioral Physiology in Germany, nailed the “time-compensated sun compass hypothesis” with another experiment in which he “tricked” starlings to misread the time from the sun’s actual position. Given that the sun changes position fifteen degrees per hour, to keep flying in a straight line using the sun as a landmark, the bird has to know what time it is in order to compensate for the sun’s shifting position. Hoffmann kept starlings in an artificially lit cage with a normal twelve-hour period of daylight, but with the lights coming on six hours earlier than actual dawn in the real (outdoor) day. These birds adjusted their activities to the artificial light schedule they experienced and expected food at a specific time in one specific direction in a circular cage, and their feeding time was of course six hours ahead of real or solar time. When his “clock-shifted” starlings were trained to expect food at their food cup in a specific direction and tested under a stationary light, they oriented ninety degrees (or fifteen degrees for every one-hour time shift) in a clockwise direction from their training dish. This experiment confirmed, by a different experimental protocol from Kramer’s, the astounding hypothesis that the birds not only use the sun as a directional compass but, like bees, also consult an internal clock to correctly compensate for its rate of movement through the sky. Clock-shifted monarch butterflies also orient in the “wrong” but predicted direction, showing that they also use the sun as a “landmark” in migration.
This sophisticated behavior of insects and birds, however, does not explain the majority of homing orientation. Most songbirds migrate mostly at night, when they could not have access to the sun’s location as a convenient directional beacon. (It is likely that small songbirds have to migrate at night because they need the daytime to replenish their energy supplies by feeding, whereas large birds, like huge airliners, have a longer flight range and burn much less fuel in relation to their body weight.) For a long time it was not known how, with neither landmarks nor sun available, the night migrants might orient. Yet orient they did, as experiments on warblers (Sylviidae) by Franz and Eleanor Sauer proved in the late 1950s.
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Bernd Heinrich
Тип: электронная книга
Жанр: Природа и животные
Язык: на английском языке
Издательство: HarperCollins
Дата публикации: 16.04.2024
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О книге: The story and science of how animals find their way home.Home is the place we long for most, when we feel we have travelled too far, for too long. Since boyhood, acclaimed scientist and author Bernd Heinrich has returned every year to a beloved patch of woods in his native western Maine. But while it’s the pull of nostalgia that informs our desire to go back, what is it that drives the homing instinct in animals?Heinrich explores the fascinating science behind the mysteries of animal migration: how geese imprint true visual landscape memory over impossible distances; how the subtlest of scent trails are used by many creatures, from fish to insects to amphibians, to pinpoint their home; and how the tiniest of songbirds are equipped for solar and magnetic orienteering over vast distances. Most movingly, Heinrich chronicles the spring return of a pair of sandhill cranes to their pond in the Alaska tundra. With his marvellously evocative prose, Heinrich portrays the psychological state of the newly arrived birds, articulating just what their yearly return truly means, to the birds and to those fortunate enough to witness this transcendently beautiful ritual.The Homing Instinct is an enchanting study of this phenomenon of the natural world, reminding us that to discount our own feelings toward home is to ignore biology itself.