Migrating Birds Reset "Compasses" at Sunset, Study Says

John Roach
for National Geographic News
April 15, 2004

Sunset is more than a thing of beauty for Swainson's thrushes and gray-cheeked thrushes. It keeps them on course when migrating between their winter and summer homes, according to an international team of scientists.

The finding sheds further light on a question that has vexed scientists for years: How do birds navigate between nesting areas separated by thousands of miles with pinpoint accuracy?

"Whatever that mechanism is, it might have great applications for navigation for mankind," said Sidney Gauthreaux, who studies bird migration, orientation, and navigation at Clemson University in South Carolina.

Over the years laboratory experiments have shown that birds orient themselves based on cues from the sun, stars, Earth's magnetic field, and by memorizing landmarks while migrating. But the relative importance of such cues was unknown and a source of scientific debate.

"In particular, nobody knew what birds are really doing in the wild," said Martin Wikelski, a professor of ecology and evolutionary biology at Princeton University in New Jersey.

To learn more, Wikelski teamed up with wildlife biologists William Cochran, of the Illinois Natural History Survey in Champaign, and Henrik Mouritsen, of the University of Oldenburg in Germany. The trio designed an experiment that allowed them to see, for the first time, how migratory songbirds behave in the wild.

The team describe their findings in tomorrow's issue of the journal Science. The research was supported by a grant from the National Geographic Society's Committee for Research and Exploration.

Wild Thing

Cochran and colleagues captured songbirds during their natural northward migration, exposed them to an altered magnetic field at sunset, and then tracked them for several nights as the birds flew through Illinois and Iowa.

The night the birds were exposed at sunset to the altered magnetic field, which was pointed east (instead of magnetic north), they flew the wrong direction—to the west. The next night the songbirds flew off in the right direction—northward.

This led the researchers to conclude that the thrushes used cues from the setting sun to update their internal magnetic compass and get back on the right track.

Gauthreaux, the Clemson University researcher (who was not involved in the study), said he is not surprised by the finding, adding that it brings some resolution to the scientific debate over which cues—visual or magnetic field—the birds use as their primary navigational tool.

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