Magazine article Science News

Mafia Cowbirds: Do They Muscle Birds That Don't Play Ball?

Magazine article Science News

Mafia Cowbirds: Do They Muscle Birds That Don't Play Ball?

Article excerpt

Cowbirds in Illinois that sneak their eggs into other birds' nests retaliate violently if their scam gets foiled, researchers say.

The brown-headed cowbirds of North America outsource nest building and chick raising. Female cowbirds dart into other birds' nests, quickly lay eggs, and rush away. The nest owners are left to care for big, demanding cowbird chicks.

Why don't the dupes throw out the odd eggs? When scientists removed cowbird eggs from warbler nests, more warbler eggs later got smashed or carried off than did eggs in nests with cowbird eggs in place. It was cowbird retaliation, conclude Jeffrey E Hoover of the Illinois Natural History Survey in Champaign and Scott K. Robinson of the Florida Museum of Natural History in Gainesville.

That's the first evidence of gangster-like behavior in cowbirds, says Hoover.

A decade of monitoring prothonotary warblers in nest boxes in southern-Illinois swamps gave Hoover the idea for the new experiment. The nest boxes sit on poles coated with axle grease to thwart raccoons, snakes, and most other raiders. Egg-laying cowbirds still strike, and Hoover had for years left the cowbird eggs alone. In 2002, he and other researchers removed cowbird eggs. Nest vandalism suddenly increased.

No one saw the vandals, but Hoover and Robinson turned to an idea put forward in 1979 by Israeli biologist Amotz Zahavi. He'd suggested that by tending the weird-looking eggs and chicks, the foster parents protect their own progeny. In a rare test of the idea, cuckoos retaliated against magpies in Spain that rejected cuckoo eggs, scientists reported in 1995. …

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