Читать книгу The Homing Instinct: Meaning and Mystery in Animal Migration - Bernd Heinrich - Страница 11

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.