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

       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.