"People thought they wouldn't want to be seen near the nest, since that might alert the host."
But the new study, which focused on nests of prothonotary warblers as the hosts, shows that the risk of retaliation by the cowbirds may be what keeps the warblers compliant.
The warblers rarely remove or destroy the cowbirds' eggs, so the researchers did the job themselves to see the cowbirds' reactions.
The vast majority of the time, the warblers' nests were destroyed soon after.
The researchers didn't catch the cowbirds in the act. But they excluded other possibilities, and the only possible culprit seemed to be the cowbirds, the scientists say.
"Because the cowbirds are freed up from raising their own young, they can spend a lot of time monitoring other birds' nests," Hoover said.
In the end, though, the compliant warblers raised more of their own offspring than those of their belligerent neighbors, the study showed.
So in the long run, the mafia behavior resulted in meeker warblers reproducing more successfully than resisters.
Insufficient Evidence?
"I find the [study's] arguments and data exceptionally clear and convincing," says conservation biologist Peter Arcese of the University of British Columbia.
Many researchers have been disinclined to accept evidence that cowbirds are destroying other birds' nests, he adds.
This reluctance "has prevented much real progress on understanding how cowbirds influence hosts ecologically and evolutionarily," Arcese said.
But other researchers still aren't convinced that the cowbirds are meting out "mafia hits."
"They don't have documentation that the cowbirds are actually [destroying the nests]," said Stephen Rothstein at the University of California, Santa Barbara.
He'd like to see the cowbirds caught on film—which study leader Hoover is planning to do in a follow-up study.
Rothstein is also concerned about the conclusions people will take from such studies.
"Cowbirds are often painted as a major factor in endangering other birds," Rothstein said. "Sometimes money goes into cowbird control instead of habitat conservation."
But even if cowbirds do hurt other birds with gangster behavior, Hoover, the study leader, agrees that habitat protection should be the focus of efforts to aid other birds.
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