Migration Mystery

Researchers discover how thrushes orient themselves on cross-country journeys

08-01-2004 // Laura Tangley

EACH SPRING AND FALL, billions of songbirds travel thousands of miles between their breeding habitats and the places they spend the winter. How the birds manage to do this so accurately-- with some returning to the same spot to breed year after year-- has long been a topic of debate. Results of lab experiments, in which potential cues are manipulated to test responses of captives, have been contradictory, with the birds' navigational skills apparently affected by several variables, including the sun, the stars, polarized light, Earth's magnetic field and spatial cues.

Now researchers have for the first time tested what happens in the wild. After attaching radio transmitters to dozens of Swainson's and gray-cheeked thrushes, they tracked the birds during spring migration. Before releasing the thrushes, the scientists exposed some to inaccurate magnetic fields (rotated 80 degrees to the east). While most birds subsequently flew north as usual, those with altered magnetic fields flew west. By the following night, however, they had corrected their course. The researchers, who published their results in Science, conclude that the birds were orienting themselves with a magnetic compass, which they recalibrated each evening based on the position of the setting sun.

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