The Mama Mafia

One nest--two types of eggs. Credit: Jeffrey Hoover and PNAS

Friday, March 9th, 2007--

The brown-headed cowbird (Molothrus ater) is notorious for laying its eggs in other birds’ nests. “Cowbirds don’t incubate eggs. They don’t raise or provision chicks. They only can leave their eggs in the nests of other species,” says Jeff Hoover an ecologist at the Florida Museum of Natural History.

This nest-borrowing behavior is well-documented. Hoover and his colleagues discovered that the cowbirds also check up on their chicks-to-be to make sure the foster parent is taking care of the eggs, Hoover reports in a study published in the Proceedings of the National Academies of Sciences.

“Even after they’ve laid an egg in a nest, they still go back and monitor these nests to see if their egg is successful or not,” Hoover says. Even more surprising, if a cowbird finds her egg missing, she is likely to come back and destroy the foster bird’s nest and eggs. Scientists call this retaliation “mafia behavior.”

“Mafia behavior” has never been documented in cowbirds, though it has been seen in cuckoos. But this may be in part because most birds never reject cowbird eggs. So the researchers set up the warblers by stealing the cowbirds’ eggs from the warblers’ nests, making it look like the warblers got rid of them. (And they call the cowbirds mafiosi!) The cowbird mothers responded 56 percent of the time by pillaging the warbler nest from which her eggs were missing. Warblers, if left to their own devices, never removed cowbird eggs.

Why would the warblers want to take care of another bird’s chick? Some researchers suspect that host birds do not recognize that the eggs are not their own. Another theory is that the host birds aren’t big or strong enough to get rid of the new eggs or that the energy expended trying to get rid of the eggs is too costly. Hoover suggests that maybe there is a different sort of cost to getting rid of the eggs: the wrath of the other mother cowbird.

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--Flora Lichtman