Correction to This Article
With a Science article in some March 19 editions, captions for photographs of a warbler and a cowbird were reversed.
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Behavior May Suggest We're Not Only Human

[Photo of a female cowbird]
Female cowbirds parasitize the nests of smaller songbirds and will damage the eggs of birds that resist. (Photo courtesy of Chris Young)

In another study, Jeffrey Hoover, an avian ecologist at the Illinois Natural History Survey in Champaign, found that cowbirds have a lot in common with gangsters.

It all began, he says, when ecologists started removing the eggs of cowbirds from the nests of warblers. Cowbirds leave their eggs in the nests of dozens of other birds. It has long been a mystery why birds such as warblers allow the eggs to hatch and why the hosts then feed the young cowbirds -- sometimes at the expense of their own offspring.

Hoover and colleague Scott Robinson found that when they removed cowbird eggs from the warbler nests, those nests mysteriously got trashed. Turns out that the cowbirds, much like members of the mob, were keeping a close eye on the nests in which they had laid their eggs. If anything bad happened to the eggs, the cowbirds would return and destroy the nest.

Hoover found that, much like the way the mafia operates, the cowbirds begin with detailed surveillance of their potential targets. That is because if a cowbird lays an egg in a warbler's nest before the warbler has laid any eggs of her own, the warbler can simply fly somewhere else and establish a new nest, and the cowbird will not be able to retaliate. On the other hand, if the cowbird lays its egg after the warbler has finished laying all her eggs over a period of three to five days, the warbler's hatchlings will emerge sooner than the cowbird's and thereby gain a size advantage in grabbing all the available food.

Hoover found that after monitoring a warbler's nest for a period, a cowbird will lay its egg in the nest right after the warbler has started laying its eggs. Cowbirds can lay 10 to 15 eggs at a time in different nests, and it appears that after laying each egg these birds then make the rounds to ensure that all the warbler hosts are toeing the line.

Cowbird eggs often look very different from the eggs of the birds whose nests they parasitize, which is why scientists had wondered why the host birds simply did not chuck those eggs. In a paper Hoover published online in the Proceedings of the National Academy of Sciences, he found that warblers that accepted the cowbird eggs produced three offspring on average. When the researchers removed the cowbird eggs from the nests, these warblers produced only one offspring on average because the cowbirds would return and trash most of those nests.

"What is interesting is the female cowbirds are running this mafia-like racket," Hoover said. "People often think of males as being violent. . . . The male cowbirds play little or no role in this."

All of this raises interesting questions. If a human playing wingman or wingwoman for a roommate is doing what the lance-tailed manakin has done for thousands of years, how much conscious thought is actually necessary for such behavior? And could the Tony Sopranos of this world plead not guilty by virtue of evolution?

Frans de Waal argues that there is a difference between cowbirds and human gangsters. A Tony Soprano knows what he is doing and understands the consequences. "The birds may not even know what reproduction is," he said. "They are not thinking, 'If I trash the nest, next time they will be careful.' "

Or . . . are they?


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