Cowbirds: Thugs of the Bird World

Image of a female cowbird. Credit: Chris Anderson

A warbler nest with 3 cowbird eggs and 5 warbler eggs. Credit: Jeff Hoover

A male Prothonotary Warbler at a nest box. Credit: Jeff Hoover

Cowbirds are the gangstas of the avian world, according to a new study that shows how these birds ransack the nests of those that don't care for their young.

Brown-headed cowbirds are infamous among scientists for being the "brood parasites of North America." They lay their eggs in the nests of other species instead of their own. The hosts then incubate the cowbirds' eggs and raise the offspring as if they were their own.

"[The cowbird] can parasitize over 100 different species commonly," said lead study author Jeff Hoover of the Florida Museum of Natural History. "By doing so, it frees up its time from having to care for offspring, incubate eggs, and it can just lay a lot of eggs every breeding season compared to other birds that have to build a nest, incubate eggs, feed nestlings and then care for the young birds after their out of the nest."

"The cowbird only has to lay eggs," Hoover said.

One of the host species that cowbirds exploit is the warbler. But should these little songsters reject the cowbird eggs, there's hell to be raised. In retaliation, the cowbirds rummage through warblers' nests in order to encourage acceptance of parasitic eggs.

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Hoover and his colleagues removed cowbird eggs from 182 parasitized warbler nests in southern Illinois and monitored them for more than four breeding seasons. The nests were ransacked 56 percent of the time when the researchers removed the parasitic egg.

"It's the female cowbirds who are running the mafia racket at our study site," Hoover said. "Our study shows many of them returned and ransacked the nest when we removed the parasitic egg."

Cowbirds also ransacked 20 percent of warbler nests that were never parasitized and 85 percent of the re-nests. This, the researchers say, is strong evidence for what is called farming behavior.

"Cowbirds 'farm' a non-parasitized nest by destroying its contents so that the host will build another," Hoover said. "The cowbird then syncs its egg-laying with the hosts' 'renest' attempt."

This research, detailed in the March 5 issue of the Proceedings of the National Academy of Sciences, verifies that cowbirds continuously monitor the nests they parasitize.

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