The Homing Instinct: Meaning and Mystery in Animal Migration

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.