Revenge of the cowbirds.

Cowbirds are famous for their wily ways.

These North American birds sneak into the nests of other birds, lay their eggs, and quickly escape before the owners return. They don't do any of the hard work it takes to raise chicks. Instead, they leave the parenting to the nest's owners.

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Given cowbirds' reputation, that's not totally surprising. But here's what might come as a shock. Even though the birds that live in these nests don't belong to the cowbird species, most of them adopt the big, hungry cowbird chicks and raise them as their own.

Scientists have long wondered why the new parents raise the greedy chicks. New evidence now suggests that sometimes the birds have little choice. If they kick the moocher chicks out of the nest, the cowbird parents seek revenge.

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For a decade, researchers from the Illinois Natural History Survey in Champaign and the Florida Museum of Natural History in Gainesville have been monitoring warbler nests in boxes in southern Illinois. During that time, they often saw cowbird eggs in the warbler nests. The nest boxes sit on poles that the scientists have covered with grease to protect the eggs and chicks from raccoons, snakes, and other predators.

Then, in 2002, the scientists tried something new. They removed the cowbird eggs from the warbler nests. Suddenly, unknown attackers started to destroy warbler eggs.

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But the damage wasn't evenly spread among the nests. The scientists found that that warbler eggs were damaged in only 6 percent of nests that held cowbird eggs. A whopping 56 percent of the warbler eggs were destroyed in the nests from which the scientists had removed cowbird eggs.

The scientists suspected that the cowbirds might be the culprits, but they needed to prove it. To do so, they removed cowbird eggs from the nests and then changed the entrance hole to keep the cowbirds from returning. When the suspected attackers couldn't get into the nests, the warbler eggs remained untouched.

Cowbirds, researchers concluded, are like mafia members: If a warbler doesn't give them what they want and raise their chicks, the parent retaliates.

It's a tough balance for the warblers. When their nests are invaded, cowbird chicks eat so much food that some warbler chicks starve. Still, the scientists say, more warbler chicks survived in this group of nests than in the group where researchers removed the cowbird eggs. Too many of those nests were attacked by vengeful cowbird parents.

Sometimes, this study suggests, it's better to put up with a bully than to try to fight back.--E. Sohn