The term "birdbrain" sounds like an insult until you learn a few things about migrating birds. Arctic terns, for example, somehow steer an 11,000-mile course each autumn from their breeding grounds north of the Arctic Circle to the antipodes of the Southern Hemisphere. They locate favorite stopovers on the Bay of Fundy, fly three days nonstop over the blank face of the northern Atlantic, negotiate the west coast of Africa, and home in on their habitual winter haunts on the Antarctic pack ice. Then, come spring, they head back north again—along a different route up the eastern coast of South and North America.
"These birds are making the longest journeys among animals on earth," says ecologist Thomas Alerstam of Lund University in Sweden.
Whether migrating or homing, birds are unsurpassed as navigators. Yet scientists still haven't found the mechanisms in bird brains that account for the birds' skill. The cues birds rely on to orient themselves aren't simple or obvious. People, for example, often use geographical cues—landmarks—to navigate. But homing pigeons can get back to their lofts from unfamiliar territory even if they're anesthetized on the outbound trip. They can find their way even while wearing frosted contact lenses that blur anything farther than a few yards beyond their beaks.
To complete their seasonal sojourns reliably, migrants need a sense of direction—what researchers call a compass—and also some idea of their position relative to the earth's surface, which is known as a map. Yet birds seem to be born just with compasses, not maps. Migrating fledglings know only to fly in a given direction for a certain number of days. Where they wind up is by default rather than design.
Experience rather than instinct seems to hone a bird's map skills. The first flight south is rough on Arctic tern tyros—less than half survive. But returning adults learn to target their southern destination with remarkable precision. They make the round-trip every year for up to 20 years or more. "This requires real navigation, not just a good sense of direction," says ornithologist Ken Able of the University at Albany in New York.
Birds could deduce both location and bearing from regional variations in Earth's magnetic field. The magnetism emanates from the planet's metallic core in lines of force that flow vertically from the poles and flatten out in the tropics. The field is most intense at high latitudes and least intense at the magnetic equator. The strength and inclination of the field create geomagnetic landmarks, and a flock of evidence supports the idea that birds navigate by these markers.
During migration time, for example, captive birds hop against the sides of their cages in the direction they would fly if they were free. German researchers Wolfgang and Roswitha Wiltschko of Goethe University in Frankfurt have shown that the hopping orientation can be modified by false geomagnetic cues. In the early 1990s, they simulated a geomagnetic crossing of the equator for garden warblers, and the birds' response to field inclination reversed—as it must in nature if trans-equatorial migrants are to stay on course. And last year, investigators at the Swedish Museum of Natural History in Stockholm induced eating binges in captive thrush nightingales by artificially re-creating the geomagnetic landscape at the edge of the Sahara. Migrating thrush nightingales must fatten up before their five-night traverse of the desert.
Birds might sense the strength and orientation of Earth's magnetic field using a pigment in the retina of their eyes. Crystals of magnetite—an iron ore—above the palate in the back of a bird's mouth could also react to magnetic stimulation. But experts say the story doesn't end there.
While geese, cranes, and storks migrate in family groups, the youngsters of most bird species don't rely on experienced kin to guide them. Many songbirds migrate individually; groups of experienced shorebirds leave the Arctic weeks before novices take off.
Since the 1950s, ornithologists have recognized that birds can use the sun as a compass, reckoning direction just as backcountry hikers do. Birds seem to "expect" the sun to rise in the east in the morning, move across the sky at an average rate of about 15 degrees per hour, and set in the west. German ornithologist Gustav Kramer of the Max Planck Institute for Marine Biology first demonstrated this principle with pigeons held in circular cages lined with feeding cups. He could train the birds to seek food from a cup in, say, the northwest part of the cage—and they would do so even if the cage was rotated or put in different surroundings. On sunny days, the birds would make a beeline for the cup, but on overcast days, the trained birds chose cups at random. In another experiment, starlings that had been similarly trained were put into indoor cages illuminated with a single stationary source of light. The birds changed cups systematically throughout the day, shifting their bearings by about 15 degrees per hour—as if the light were tracing out an east-west arc that governed their orientation.
Subsequent studies have shown that birds can't use the solar arc as a map. So it has become clear that orientation involves a complex of cues and sensing systems rather than a single, master governor. Birds deploy the system that's likely to work best under the circumstances. On sunny days, for example, homing pigeons use their sun compass, but on overcast days they seem to consult their magnetic sensors instead. Birds that migrate at dawn and dusk, like robins, probably get their bearings from a band of polarized light that runs north to south across the sky at sunrise and sunset. Italian biologist Floriano Papi of the University of Pisa has argued for decades that pigeons construct olfactory maps from odors borne on prevailing winds. Wind direction itself can serve as an orientation cue. Many songbirds migrate at night using a star compass set by celestial rotation. Birds may even be able to recognize certain stars.
"Birds, like people, use a whole variety of cues to find their way," says neurobiologist Charles Walcott of Cornell University, who has conducted many definitive studies of homing-pigeon behavior. "Which they use depends on which ones are most useful in their environment."
Thomas Alerstam's recent studies of Arctic shorebirds reveal the virtues of a flexible navigation system. He used radar to track migrating flocks of sandpipers and plovers as they left their breeding grounds for points southeast. The birds' trajectories were clearly dictated by the sun compass—which makes sense because steep magnetic-field lines send disorienting signals near the poles, and stars are rarely visible in the polar summer. But Alerstam also discovered that the birds' paths resemble great-circle routes: the curved shortcuts that jet planes follow to minimize east-west flight times at high latitudes. The curving, says Alerstam, results from a kind of jet lag induced by nonstop east-west flight. Because some of the migrants cross four time zones in just a day or two, their internal clocks fall out of sync with the sun's schedule, and their compass bearings get skewed. "They make an orientation error of four hours, up to 60 degrees," Alerstam says. "This is a way birds exploit the 'weakness' in the sun compass to their own advantage."
The shorebirds probably switch to other cues as they travel down the east coast of North America, Alerstam says, because great-circle shortcuts don't exist for north-south flight paths, and the sun's angle gets hard to read near the equator. "At tropical latitudes, I would advise birds not to use the sun compass."