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Birds Follow the Sun

12 January 2001 7:00 pm
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On track. These flight trajectories are predicted for birds using geographic compass directions (blue), geomagnetic fields (green), or a sun compass (red). Migration routes best match those shown in red.

Following the sun imperfectly turns out to be the perfect solution for navigation if you're a bird in the Arctic, researchers have found. By using the sun to orient themselves, tundra-breeding shorebirds end up approximating the "great circle routes" used by airplanes and ships--routes that minimize the distance between two points on the surface of a sphere.

Birds can travel thousands of kilometers and arrive at breeding and wintering grounds with pinpoint precision. Fascinated, biologists have investigated this ability for decades. They've learned that birds take their bearings from such things as stars, the sun, landmarks, and Earth's magnetic field. But existing ideas didn't explain why some birds in the Arctic fly east before heading south. So ornithologist Thomas Alerstam of Lund University in Sweden and colleagues used the radar on a Canadian icebreaker in the Northwest Passage to measure the direction of migrating birds flying past.

When Alerstam and his colleagues plotted the routes taken by shorebirds, they found that migrating plovers and sandpipers were curving ever more southward as they flew east. The researchers ruled out other orientation cues and discovered the birds were using a sun compass, they report in the 12 January issue of Science. But the birds' internal clocks can't keep up with their nonstop movement, apparently, and they become out of phase with local time. By failing to compensate for their movement across time zones, they misread the sun's position and veer increasingly southward. But this fortuitous mistake allows the birds to fly south in trajectories approximating the great circle routes that minimize travel distance, saving them valuable energy.

The field study "represents a big leap," says Sidney Gauthreaux of Clemson University in South Carolina, by adding to knowledge gained from lab experiments. He and other ornithologists agree that the uncorrected sun compass strategy works best near the poles, and that additional cues are probably involved at lower latitudes.

Related sites

The Alerstam lab home page

Images from the expedition

A primer on shorebird migration

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