The second question is: How is an aggregation formed?
Insects have an impressive ability to home in on scent, and ladybird beetles could find the aggregation by following an odor plume. Memory cannot be involved in the ladybugs that Otter found year after year aggregating at the same site, which were generations removed from those of the previous year. The beetles arrive on site in September, stay there eight months, and in May return to the valley floor to feed, mate, and reproduce. Only their descendants, two or three generations later, could return to the tiny spot where they had hibernated.
The aphids’ rule of flying up toward the light, to then be dispersed by the wind, and then homing in on green when they settle could be a model of what happens in ladybird aggregation. Ladybirds are much stronger fliers than aphids, although they too are swept along in updrafts. But such drifting, while helping to account for their annual ascent from the lowlands into the mountains, does nothing to explain how tens of thousands of them end up under the same rock pile.
If the ladybirds home in on color, this could be tested, as the aphids’ homing in on food plants was tested—by leaving color targets at sites other than the traditional hibernaculum. But even if red color is an attractant (highly unlikely because the beetles aggregate under the rocks, not on them), that still would not explain their annual return to the same place in successive years. Do they smell their ancestors? Could thousands of smelly beetles piled up for eight months leave sufficient scent residue to serve as a marker that allows others to home in on the spot? If so, this idea could also be tested, by transferring an aggregation of beetles to overwinter at another physically similar place in the same general area as the old to see if a new traditional homing site for their mutual protection is created.
By the Sun, Stars, and Magnetic Compass
Life has unfathomable secrets. Human knowledge will be erased from the world’s archives before we possess the last word that a gnat has to say to us.
—Jean-Henri Fabre
CHARLES DARWIN REFERRED TO THE ACCOUNT OF FERDINAND von Wrangel’s Arctic explorations, The Expedition to North Siberia, concerning how we home, quoting von Wrangel on how the Siberians oriented by “a sort of ‘dead reckoning’ which is chiefly affected by eyesight, but partly, perhaps, by the sense of muscular movement, in the same manner that a man with his eyes closed can proceed (and some men much better than others) for a short distance in a nearly straight line, or turn at right angles and back again.” Darwin compared a bird’s homing capability with that of people, but much less favorably, by telling how John James Audubon kept a wild pinioned goose in confinement, which at migration time became “extremely restless, like all other migratory birds under such circumstances; and at last it escaped. The poor creature immediately began its long journey on foot, but its sense of direction seemed to have been perverted, for instead of traveling due southward, it proceeded in exactly the wrong direction, due northward.” I’m not the least surprised at the behavior of the goose but all the more puzzled by our own orienting, which involves knowledge versus a feeling of “sense of direction.” I recall instances of waking up in “total” dark, “knowing” in my mind precisely how I am oriented relative to the room and hence the rest of my environment, but irked by “feeling” that I am in the precise opposite direction. It is then a struggle to get the two to agree, which happens only after some effort.
In Darwin’s time it was still supposed that humans had overall superiority over other animals. His then-hypothesis (later theory, and now fact) of evolution, which now binds us all as kin, was still revolutionary. Darwin found the goose’s behavior puzzling because he could not know that geese, cranes, and swans stay together in family and larger groups and that although the young by themselves do not know the correct migration route, they learn to know it from their parents which in turn learned it from theirs. Other birds have their migratory directions genetically coded, and they go strictly by “feeling,” since many of these have no knowledge because they migrate ahead of their parents.
We humans get lost easily. We would not get far without reference to landmarks, and I base that conjecture on (inadvertent) experiments. In one I was in long-familiar woods and got caught in a heavy snowstorm. Suddenly I got “turned around,” and it seemed as if all landmarks had in almost one instant been erased. But I kept going, trying to maintain a straight line by trusting my “sense of direction.” When I thought I had reached a place that I knew, where I should be going downhill, the landscape was instead sloping upward, and the brook I had expected to see was going in the “wrong” direction. At that point, knowing I was lost and no longer referencing to any signal, I backtracked in the snow and discovered that I had been walking in a circle, all the while thinking I was heading in the “right” direction. Yes, we can walk, a short way, in a relatively straight line with our eyes closed, by a process dubbed “path integration,” but my emphasis here is on “relatively.” Mice may do better. A friend told me of catching a Mexican jumping mouse in a live trap baited with peanuts. It had a kink in its tail, so he called it Crooked Tail, or CT for short. After he had released it several times, and it always returned for another snack of peanuts at the same source, he finally decided to “test its mettle” and released it exactly one kilometer away in 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 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 sout
h. 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.
The Homing Instinct Page 7
