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

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