Incentives matter even among birds: threats to destroy nests ensure that other birds raise cowbirds' offspring

Raise my kids, or else! People have long wondered how cowbirds can get away with leaving their eggs in the nests of other species, who then raise the baby cowbirds. Why don't the hosts just toss the strange eggs out? Now researchers seem to have an answer _ if the host birds reject the strange eggs, the cowbirds come back and trash the place.

The so-called "Mafia behavior," by brown-headed cowbirds is reported in this week's online edition of Proceedings of the National Academy of Sciences.

"It's the female cowbirds who are running the mafia racket at our study site," Jeffrey P. Hoover, of the Florida Museum of Natural History and the Illinois Natural History Survey, said in a statement.

"Our study shows many of them returned and ransacked the nest when we removed the parasitic egg," he explained.

Hoover and Scott K. Robinson of the Florida museum studied cowbirds over four seasons in the Cache River watershed in southern Illinois.

While cowbirds leave their eggs in many other birds nests, the researchers focused on warblers in the study because warblers usually accept and raise cowbird eggs.

To see what would happen, Hoover and Robinson watched where the cowbirds left eggs in warbler nests, and then removed some of them.

They found that 56 percent of the nests where cowbird eggs were removed were later ransacked.

They also found evidence of what they called 'farming' behavior,' in which cowbirds destroyed a nest to force the host bird to build another. The cowbird then synchronized its egg laying with the hosts' 'renest' attempt.

"Cowbirds parasitized 85 percent of the renests, which is strong supporting evidence for both farming and mafia behavior," Hoover said. . . .