Songbirds can migrate for thousands of kilometers at night and in poor weather yet still end up in the right place. Now, in a logistical tour-de-force, three researchers have revealed how the birds get their bearings before takeoff each night: They set their course according to a magnetic compass they calibrate at sunset.
Most of what's known about bird navigation comes from experiments with caged birds. Researchers place them at the base of a vertical funnel, then alter the magnetic field, the orientation of polarized light, or other cues. By watching which direction the birds try to hop out, researchers try to discern which sense the birds rely on most. But these "cue-conflict" studies have yielded conflicting results.
A major question was what happens in the wild. William Cochran, retired from the Illinois Natural History Survey, Martin Wikelski of Princeton University, and Henrik Mouritsen of the University of Oldenburg in Germany conducted an experiment to find out. They captured several dozen gray-cheeked thrushes and Swainson's thrushes near Champaign, Illinois, and glued small radio transmitters to them. Before releasing the birds, they exposed some to "false" magnetic fields, rotated 80 degrees to the east, during sunset.
The researchers followed the birds as they flew through the night, tracking them with a meter-tall antenna mounted on top of a battered 1982 Oldsmobile. "It's quite a chase," Wikelski says. Control birds flew northerly, but those that had been in the altered magnetic field flew westward for the entire night. The next evening, after sunset, the experimental birds corrected their course and headed north.
The birds seem to be calibrating their compass to the sunset, perhaps from the position of the sun or the pattern of polarized light it creates in the sky. They may then set their course by comparing the twilight cues to the orientation of the magnetic field lines. Calibration would help navigation, because magnetic field lines vary from place to place and don't always point toward true north.
The new study is "extraordinarily straightforward and convincing," comments John Phillips of Virginia Polytechnic Institute and State University in Blacksburg. However, it's not certain that once airborne, the birds actually fly by their magnetic compass, Phillips notes; they may use it to get their bearings at dusk, then keep their eyes on the stars instead.