How migrating birds find their way over great distances has long intrigued people. Some birds can orient themselves with an internal compass using the earth's magnetic field. Others seem to follow the sun, the stars, polarized light or different clues.
Martin Wikelski, a professor of ecology and evolutionary biology at Princeton University, and colleagues studied thrushes flying north through Illinois on their migration from South America to Canada. These birds fly at night, guided by their ability to sense the magnetic field of the earth to keep them flying in the right direction.
The researchers set up an artificial magnetic field - oriented differently from the earth's magnetism - around birds in outdoor cages, according to a report in Friday's issue of the journal Science.
The birds were fitted with tiny devices that allowed the scientists to radio track them and, once it was dark, the birds were released.
Most set off on their migratory trek, but headed west instead of north, flying several hundred miles in the wrong direction. The next day, however, they corrected themselves and went north again, Wikelski said. The birds somehow had reset their internal compass now that they no longer were in the artificial magnetic field.
They did not compensate for the distance they had gone westward, Wikelski said, but did reorient to fly north until they reached a certain latitude. Then they flew east and west looking for the familiar breeding grounds, he said.
Birds have enormous economic importance, eating billions of insects, and they can also be carriers of disease, such as West Nile virus, Wikelski said. So it is important to know where they go on their travels and to learn how they find their way.
Charles Walcott, former director of the ornithology lab at Cornell University and now a professor of neurobiology and behavior, said the research on thrushes "seems to be wonderfully clear in suggesting they use the magnetic field as a reference and are, in fact, calibrating it with some kind of clue at sunset."
He said the report does not indicate what that clue is, whether the direction of sunset, polarized light, or something else. Walcott added that what is true for thrushes is not necessarily the case for other birds because various breeds may use different systems of navigation.
Peter P. Marra, an ecologist at the Smithsonian Environmental Research Center in Edgewater, Md., said it is "just amazing that they can follow the migrating birds that long and do the experiments that they have conducted."
Marra said it is fascinating that the researchers "have been able to show that the stars are not playing as important a role as people have thought before and that it is this magnetic compass that is allowing these birds to reorient every day and get them in the right direction."
By Randolph E. Schmid