updated 8/5/2004 11:12:21 PM ET 2004-08-06T03:12:21

It's a lesson many little humans could learn from baby birds: Sometimes, being nice to other youngsters pays off. Brown-headed cowbirds, like several other bird species, leave their eggs in the nests of other birds, who then feed and raise the cowbird chicks.

In many of these so-called social parasite species the new arrivals push eggs already there out of the nest, or kill chicks already in the nest.

But the little cowbirds don't always do that, and Rebecca M. Kilner of the University of Cambridge, England, wondered why.

It turns out that when a little cowbird shares the nest with baby birds already there, it grows faster and stronger than when it is the only chick.

The study is being published this week in the journal Science.

One possible reason, the researcher said, is that, "collectively they evoke a higher level of provisioning than the parasite could ever achieve alone."

All in the family
In other words, with more mouths to feed, the parent birds work harder, even though one extra mouth is a stranger in the nest.

The researchers found that with three hungry babies, two flycatchers and one cowbird, parent flycatchers averaged 36 feedings per hour, compared with 14 when there was only a baby cowbird.

Kilner and colleagues studied 20 nests of Eastern phoebes, a type of flycatcher, in Tompkins County, N.Y.

In 10 nests they removed the flycatcher eggs and put in one cowbird egg, while in the other 10 they put in the cowbird egg but left two flycatcher eggs, or chicks hatched the same day as the cowbird.

They then weighed and measured the chicks daily. After eight days the social cowbirds weighed 14 percent more than the loners.

"It's as if the host young help the cowbird ... but they get as much as they would without cowbirds," Kilner said.

The flycatcher chicks didn't benefit from the arrangement, she said, but they didn't suffer either, though the cowbird chicks managed to gobble down 56 percent of the food brought to the nest.

This discovery opens a great can of worms, Kilner said.

There are many species of parasitic birds and about half kill the eggs or chicks of their hosts.

"Now we have to explain why (behavior) differs," she said.

Finding that out, Kilner said in a telephone interview, "has general bearings on our understanding of social evolution."

"One puzzle is why animals live in groups when we think of them as being selfish," she said. "The increasing evidence is animals can benefit from living in groups, even if not with relatives, so we show cowbirds can benefit by sharing the nest."

The research was supported by the British Royal Society, the United Kingdom Natural Environment Research Council and the Miller Institute for Basic Research in Science, University of California, Berkeley.

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