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Sunsets keep songbirds from getting lost

A Swainson's thrush. Image courtesy of Laura H. Spinney.
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Night-migrating songbirds use sunsets to help them fly back and forth between winter feeding grounds in Central and South America to summer breeding grounds in North America.

A Gray-cheeked thrush, Catharus minimus. Image Science.
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The scientists report that two kinds of songbirds, Swainson's and gray-cheeked thrushes, rely on the sun, which always sets in the west, to help them update their internal compasses. These magnetic compasses help them locate the final destination of their long migrations.

The scientists captured birds during their northward migration and attached temporary tracking devices using false eyelash adhesive. For several nights, the scientists tracked the wild thrushes as they migrated through the states of Illinois and Iowa, in the center of the United States.

This station wagon, with a bird-tracking antenna, is part of the fleet of automobiles scientists used to follow the birds. Image Science.
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A police officer in a small town in Iowa stopped one of the "suspicious looking" bird-tracking cars around three in the morning. Study author Martin Wikelski from Princeton University was in the car. He explained to the police officer that the antennas, computers and other gadgets help them follow the birds through the night. The officer quickly let them get back to bird tracking.

Hours before they were pulled over, the scientists exposed thrushes they captured to confusing magnetic field lines. These magnetic field lines went in a different direction than the lines from Earth's magnetic field that the birds usually follow. The scientists hoped the turned magnetic fields would mislead the birds, and their plan worked.

Most birds flew west all night when they should have been flying north. But the next night, after a day of rest and a sunset, the thrushes flew north again.

The authors conclude that the Swainson's and gray-cheeked thrushes used the sunset to get back on track. Thrushes seem to steer by a magnetic compass when migrating at night. They update their magnetic compasses using the setting sun. This sunset strategy minimizes confusing shifts in direction of the Earth's magnetic field in northern areas where these songbirds breed. Many other songbirds probably also use the sunset as they migrate, the authors say.

This study appears in the 16 April 2004 issue of the journal Science.


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