"Mafia Birds" Make Others Raise Their Young ... Or Else

Mason Inman
for National Geographic News

March 6, 2007

Birds can act like gangsters who threaten shopkeepers who fail to pay "protection money," a new study says.

The study focused on the brown-headed cowbird. Females of the species lay eggs in the nests of other bird species, leaving the strangers to incubate and raise the cowbird chicks—or have their nests attacked.

(Related: "Raised by Others, Cowbirds Use Code to Find Their Kind" [October 15, 2004].)

Brown-headed cowbirds "can lay their eggs within ten seconds," said avian ecologist Jeffrey Hoover of the Illinois Natural History Survey, who led the new study. "It's quite amazing how fast they are" compared with other birds.

This cowbird, the main "parasitic" bird species in North America, depends completely on other birds to raise its young—or else.

Cowbird mothers keep watch on the nests where they've laid their eggs.

If the birds find that their eggs have been destroyed or removed from the nest, the cowbirds retaliate, the study says.

The birds reportedly destroy the host birds' eggs, pecking holes in them or carrying them out of the nest and dropping them on the ground.

The study appears this week in the journal Proceedings of the National Academy of Sciences.

Losers Really Do Win

It has long been a mystery why the host birds put up with this exploitation.

"The conventional wisdom has been that [the cowbirds] didn't go back and monitor the nests" where they'd laid their eggs, Hoover said.

"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.

Free Email News Updates
Best Online Newsletter, 2006 Codie Awards
Sign up for our Inside National Geographic newsletter. Every two weeks we'll send you our top stories and pictures (see sample).