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

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СКАЧАТЬ 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.

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