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Planet Earth

Whirlybird

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While resting on salty inland lakes and along seashores during its winter migration from the Arctic to the west coast of South America, the red-necked phalarope has been spotted spinning in circles for minutes at a time. Biologists have known that the twirling uses a lot of energy and that it seems to help in catching food, but it was never clear exactly how it helps. Now ornithologist Margaret Rubega at the University of Nevada at Reno and her colleagues have discovered that the twirling motion creates a water current that not only concentrates prey like brine shrimp but also brings out-of-reach food closer to the surface. Apparently this ducklike marine bird hates to go underwater. They never dive, says Rubega, unless they’re being chased by some nasty predator. The phalaropes create the current by kicking one leg harder and faster than the other, which causes them to spin at about one rotation per second. This moves some surface water aside and brings deeper water up to replace it, creating a miniature upwelling. Individual birds apparently have favorite tactics; some always spin clockwise, others always counterclockwise. Usually, when food is already at the surface, the birds swim along and pick at the water. But when they see prey that’s too deep to snatch with their beaks, they float over it and whirl away. They know exactly what they’re doing when they do it, says Rubega.

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