The Homing Instinct: Meaning and Mystery in Animal Migration. Bernd Heinrich

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.