"The magnetic field or magnetic direction may be perceived as a dark or light spot which lies upon the normal visual field of the bird," Heyers said, "and which, of course, changes when the bird turns its head."
The study was published in a recent issue of the Public Library of Science journal PLoS ONE.
More Navigational Tools
Scientists not involved with the study said it is impressive and well done, but cautioned that there are more pieces to the puzzle of how birds navigate on their long migrations.
"An animal that has to migrate over great distances needs to have both a compass and a map," said Cordula Mora, a biologist who recently completed her postdoctoral research at the University of North Carolina, Chapel Hill.
Mora's work suggests that birds may use magnetic crystals in their beaks to sense the intensity of the magnetic field and thus glean information on their physical location. (Related news: "Magnetic Beaks Help Birds Navigate, Study Says" [November 24, 2004].)
"If you have a compass, you know where north, south, east, [and] west [are], but you don't know where you are, so you don't know where you should be going," she said.
Study author Heyers said "both [map and compass] systems may act in concert."
Robert Beason is a wildlife research biologist with the U.S. Department of Agriculture in Sandusky, Ohio, and an expert on bird navigation.
He noted that stars may also either fully or in part provide the birds with their visual bearing—not the magnetic field.
The next step is to figure out where all this information comes together in the bird brain, he noted.
"That's probably going to tell us where the navigation center for birds is," he said.
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