The magnetic compass orientation hypothesis was almost concurrently confirmed for a small European migrant, the European robin, Erithacus rubecula. Instead of altering the magnetic field that the birds carried, Wolfgang and Roswitha Wiltschko from the Max Planck Institute for Ornithology in Germany altered the magnetic field around cages holding birds. The migratory-ready robins responded to those changes appropriately as expected if they oriented to Earth’s normal magnetic fields. Since then, hundreds of studies show magnetic orientation to be so common as to be almost universal in a great variety of animals, although magnetic orientation and the sun compass are only two of multiple cues used in multiple ways.
Not far from Emlen’s Cornell University in Ithaca, New York, the team of Kenneth and Mary Able found in the late 1990s that birds, in this case specifically savannah sparrows, Passerculus sandwichensis, a typical night-migrating passerine, use magnetic, star, and polarized light cues, and possibly also the sun, for determining and maintaining homing direction. But their preferred orientation reference is the magnetic, and even young hand-reared birds that have never seen the sky orient south in the fall by using the magnetic lines of force. However, visual cues (from stars and polarized light) can then be calibrated to the magnetic compass. In this case, information from star patterns and polarized light overrides the magnetic compass orientation. The sparrows learn to orient secondarily to the pattern of polarized light that varies in relation to the sun’s position, and that is especially prominent at sunset. This system now appears to apply generally for songbirds, as confirmed by more recent research by William W. Cochran and colleagues using free-flying radio-tagged gray-cheeked, Catharus minimus, and Swainson’s, C. ustulatus, thrushes.
Thrushes in spring migratory condition were subjected to an experimentally altered eastward-turned magnetic field during twilight (they were caged at that time before their release), a period when they naturally orient in a westerly direction. On release after dark (now in the Earth’s normal magnetic field), they flew westward—although normally without the prior treatment they would have flown north. As predicted from their twilight eastward-turned orientation, they continued flying in the same (wrong) direction the whole night. But on the next and subsequent evenings, the radio signals tracking them showed that they had changed direction, now flying in the “correct” northerly migratory direction. The birds apparently take their direction cues from the polarized light patterns in the sunset direction, to recalibrate their magnetic compass direction specifically just before takeoff during each twilight. Thus, even when they migrate through areas of the Earth with perhaps local magnetic anomalies, they are not thrown off course.
The physiology of how the magnetic sensing works is still a mystery. Possibly iron-containing minerals align like a compass and cause mechanical deflection that is sensed. Presumably the cells or something in the cells swivels like iron filings do in response to a magnet. The mechanical displacement might then cause cellular depolarization, much like what happens to mechanoreceptors at the base of hairs when they are bent, as happens during hearing. Such a receptor has been claimed to be located in the snouts of trout, and in the upper bill of pigeons, while others suspect the magnetic information comes from the ears. Still another model being developed to explain magnetic information is linked to the visual system, and this one potentially provides vastly more information.
Animals “seeing” a map of the magnetic landscape may be more literally true than just hypothetical. Evidence is mounting that some birds can sense magnetic information optically—that the optic nerve transmits changes in magnetic fields to the brain, and light affects the responses to magnetic fields. That is, it appears that birds may “see” magnetic lines of force. What this means in terms of images is still unknown and would be difficult for us to imagine, although the evidence indicates that the magnetic lines of force are not seen in the way we read a compass, namely, along the directional component. Animals may additionally see magnetic lines of force along the vertical component. In the north, for example, the lines of force of the Earth’s magnetic field are strongly directed upward away from the pole, at the equator they are nearly horizontal, and in the south they point downward toward the pole. Therefore, birds’ ability to see lines of force gives them the potential to determine latitude.
More recently, the Wiltschko team examined a species of migratory Australian silvereyes, Zosterops lateralis. At migration time, their caged birds oriented predominantly in the northerly migratory direction. As expected, the birds reversed their headings from north to south when the vertical component of the magnetic field around them was reversed from the ambient normal. However, light intensity and light wavelength had a huge effect also; if the birds in the lab were put into bright blue light, they oriented along the east-west compass direction, and under green light they reoriented toward west-northwest. This result is puzzling and shows that there is more going on than we understand, especially since the directions taken in response to light changes were not reversed by inverting the polarity of the magnetic field.
A series of other very recent studies are delving into how the magnetic compass of birds is related to light-sensitive pigments in their retinas, since some of these pigments respond to a bird’s alignment in a magnetic field. Such light- and magnetic-sensitive pigments are found in retinal cells of some migrants, but not in nonmigrants, and the neurons connected to these pigment cells show high levels of activity when the birds are orienting by magnetic information. Furthermore, recent studies of European robins by Katrin Stapput and others of the Wiltschko team indicate that specifically the right eye may play a dominant role. The robins were equipped with goggles; those with a frosted goggle that blurred their vision in the left eye oriented correctly to magnetic information, but those with vision from the right eye experimentally blurred did not. Thus, the observations in aggregate raise the possibility that birds may literally see ghost images of the Earth’s magnetic field superimposed on their perception of images of objects.
How animals orient is now being examined also from a neurobiological perspective. Recently the team of Le-Qing Wu and David Dickman from the Baylor College of Medicine discovered fifty-three neurons in the pigeon’s brain stem that respond to the strength and direction of the surrounding magnetic field. Although the sensory organ from which these cells receive the magnetic information remains unknown, the researchers suspect it is the inner ear. The upshot of this rapidly expanding field is that rodents and humans have specialized neurons, called “grid cells,” that in the brain produce localized electrical activity that shows up in functional magnetic resonance imaging (fMRI). These grid cells appear in localized areas that coordinate behavior, and such “cognitive maps” inscribe the directions and speed of movement of the animal. Some animals apparently carry a neural representation that keeps track of where they have been and are, in part by how fast they are going and hence how far they have traveled. Indeed, this neural representation can, in humans, be duplicated in a virtual reality arena that mimics the actual spatial movements.
The mechanisms I’ve described so far assume that the animals are adapted to a static world, one with dependable, though complexly changing, cues. But although the angle to the sun or to the North Star pattern is relatively constant, the homes of the animals in relation to them are not. Recent work with blackcap warblers in Europe shows a dynamic picture of rapid evolutionary change. Blackcaps are common breeders throughout northern and central Europe from where they traditionally migrate directly south to the Mediterranean and to North Africa in the winter. They nest in low garden hedges where they are conspicuous singers, and people welcome them each spring after their long migrations back north.
In the 1950s and 1960s, increasing numbers of the blackcaps were found to spend their winters in Britain and Ireland, where they had never been seen before at that season. Since blackcaps do breed in the British Isles, it was assumed that the winter-present birds there were simply some summer breeders th
at had stayed rather than migrating, because people had provided sustenance via bird feeders. But then on December 14, 1961, due to a cat and coincidence, the story suddenly got complicated. A resident in Ireland picked up a blackcap that a cat had just dragged in and noted that this bird carried a metal band on its leg. The band identified the bird as originating in Austria, and the finder sent it to the Austrian ornithological society.
The cat’s catch strengthened a suspicion of Peter Berthold, of the Max Planck Institute for Ornithology in Radolfzell in southern Germany, that the blackcaps wintering in Britain might actually come from breeding grounds in central Europe. He and colleagues then conducted a historic study. Berthold caught blackcaps wintering in Britain, transported them to the Max Planck research labs, and kept them there until the fall when they were ready to migrate. He then tested them in cages and found that these birds, when in Zugunruhe, oriented westward, in the direction of the British Isles. In contrast, the locally breeding German blackcaps, which migrate to the south, oriented in his cages in that direction. Berthold then found that the respective direction the blackcaps choose is innate; it is determined by genetic programming. He crossed blackcaps bred in Germany that migrated south with those that also bred in Germany but overwintered in Britain, and these caged hybrids oriented at migration time in an intermediate, southwesterly direction (if they had been released they would likely have perished, because they would have landed on the Atlantic Ocean off the coast of France).
Continental European blackcaps overwintering in Great Britain, which apparently started as a rare phenomenon in the early 1950s, are now common. Tens of thousands of blackcaps now migrate each fall from their breeding grounds in central Europe to new winter homes, thousands of gardens throughout Britain and Ireland. The few initial blackcaps, which due to a genetic mutation initially flew to Britain rather than south from central Europe, had by chance come into a new home where they prospered, and then they multiplied.
Berthold suggests that the advantage of the mutation that resulted in the birds’ migrating to and from Britain from central Europe could relate to the fact that the new migration route is fifteen hundred kilometers shorter than that of the south-migrating blackcaps. Perhaps the birds arrive on their breeding grounds in central Europe from Britain earlier and more rested than the longer-range migrants wintering at their traditional sites in Africa, and so they could pick and defend the best breeding territories before their competitors, those from the southern migration, arrived. Alternately and additionally, perhaps the wintering grounds in Britain are now more advantageous because of climatic changes and more bird feeders. The larger picture is clear: evolution of a new behavior has occurred almost instantaneously.
On reflection, these kinds of changes in homing behavior must have occurred in the past and probably still happen routinely. The birds are using magnetic orientation, and Earth’s magnetic fields flip on average once every half-million years. These magnetic reversals are revealed in dated iron-containing rock that, as it hardened, “froze” the magnetic particles in it into the place of the then-existing Earth’s magnetic field when they were still free to move in the molten material. The now-hardened rock that has erupted from the Earth’s interior in the gradual spreading of the mid-ocean ridges reveals the record of the magnetic reversals that have been occurring for at least the past 150 million years. A magnetic flip can occur in the span of a century, and havoc in orientation may result, because suddenly all the “rules” have changed. Similarly, only ten thousand years ago, central Europe was covered with glaciers. There would have been no blackcaps at home there. When the birds invaded the cleared lands after the glaciers left in order to nest there, they would have had to migrate out in the winter and back in the spring. Relatively precise migratory directions had to be inscribed into their genes to ensure precise homing. The outliers were the safety valve from what would otherwise have been a straitjacket of inflexibility. As the landscape changed, those that changed with it prospered.
But why do those blackcaps that nest in Britain and Ireland still migrate? Why don’t they just stay? Perhaps eventually they will. Perhaps a small mutation can change a migratory direction, but it would take a much longer time to eliminate migration behavior entirely because that behavior involves many responses and is therefore probably more deeply encoded. Migration as a whole involves timing, the restlessness to motivate long-distance flight, feeding binges, and sometime extreme fat deposition. All of this must require a large genetic program that likely cannot be shed by a one- or two-gene mutation. One thing we can be sure of is that the group of blackcaps that now spends the winter in Great Britain came from a small founder population.
Long-distance feats like the shearwater’s, and those of other oceanic birds like albatrosses, became well documented, although the homing mechanisms were long a mystery. But before delving into the one study that chips away at the mystery of albatross homing, let us look at the homing of turtles, a far more “primitive” group of animals. Turtles are far older in evolutionary terms than birds. They are far older still than the dinosaurs and their relatives that last roamed the Earth and the seas sixty million years ago. Turtles in their present form were around over three times longer than that—about two hundred million years ago.
The late Archie Carr, a zoologist at the University of Florida, was one of the first persons to delve into turtle homing ability, and since 1955 he devoted his life to studying the green turtle, Chelonia mydas. Carr became the world authority on these reptiles and left a large legacy. His extensive tagging studies of turtles revealed a population that fed along the coast of Brazil and nested on Ascension Island, a five-mile-in-diameter pinpoint of land in the vastness of the Atlantic Ocean that is more than twenty-two hundred kilometers from the green turtles’ feeding grounds. Mating takes place at the nesting grounds, and every two or three years the turtles make the trip from their feeding pastures near Brazil to their breeding place on Ascension and then back again. Carr realized that “this population seems to have evolved the capacity to hold a true course across hundreds of miles of sea—the difficulties facing such a voyage would seem insurmountable if it were not so clear that the turtles are somehow surmounting them.” He believed that the ability to make the long sea voyages without landmarks, the capacity for open-sea orientation, is “the ultimate puzzle” in the study of animal homing. Although he deduced that the turtles “must have some kind of compass sense” that guides them in the open seas, he didn’t know what senses were involved either in maintaining a straight line of travel or in identification of place.
Carr realized that to test for that “compass sense” he had to track the turtles’ paths in the open ocean, much as Menzel had recently done with honeybees, equipping them with transponders and tracking them by radar over the countryside. Carr initially tracked turtles by having the animals tow a helium-filled balloon that would be visible even when the submerged turtles were not. But this method required staying with the turtles throughout their whole journey, and it would not be humane to leave the balloons attached indefinitely. He then tested the feasibility of equipping turtles with a radio transmitter. In 1965 Carr announced, “It may soon be possible for turtles to bear a radio transmitter and a power source” and “each time a satellite passed within range of the towed transmitter a signal would be received; these signals, rebroadcast to a control station, would allow a precise plotting of the position of the turtle.” In this way the question of whether or not the turtles’ homing resulted from mere improbable chance or true navigation could be answered. As in research with other animals, lab tests are almost always needed to complement field tests.
David Ehrenfeld, a biology professor at Rutgers University, equipped young green turtles with glasses in which only one of the two lenses was made opaque. Turtles so outfitted were lost: they kept moving in tight little circles in the direction of the eye that could see. Yet since the animals cannot see their thousands-of-kilometers-distant target as an image in
the open ocean, they may instead see directions. Sea turtles are, in fact, notoriously myopic, although they orient to light and dark. It now looks as though vision may play a role not only in turtles’ close-in detecting of images such as food, but also in their detecting of magnetic fields.
Insights gleaned from birds could later be reapplied to the problem of sea turtles’ orientation. As Archie Carr had posited decades earlier, turtles must depend on some kind of compass orientation, even though by itself this capacity could not explain homing. Homing requires not only a compass, but also a “map.” However, the team of Kenneth and Catherine Lohmann from the University of North Carolina have in the past two decades provided further insights into the homing mysteries of a sea turtle, the loggerhead, Caretta caretta, a species that nests on beaches in eastern Florida, returning there after swimming about fourteen thousand kilometers.
After loggerhead young hatch and emerge from their nest in the sand at night, they dash directly for the beach, guided by moonlight reflected off the water. Then, guided by the pattern of the waves after reaching the water, they attain the currents of the Gulf Stream, and then the Atlantic Ocean gyre takes them in a giant circuit into the Sargasso Sea. It is a grand migration of nearly ten thousand kilometers, at the end of which the one in thousands that survives to adulthood returns about twenty years later to a place near its home beach.
The Homing Instinct Page 9