by Whit Gibbons

August 7, 2011

Q. Do cowbirds deposit their eggs in the nests of other cowbirds?

A. No. Cowbirds, like European cuckoos, are "obligate brood parasites," which means they lay eggs in nests of birds outside their own species. In fact, European cuckoos and cowbirds no longer even construct their own nests. (Nonobligate brood parasites lay eggs in the nests of other birds in their species, as well in their own nests.) At least 100 species of birds are known victims of the cowbird's sneaky behavior.

Cowbirds are native to areas of the United States where the buffalo once roamed. They made a living by eating insects stirred up by the herds as they moved across the landscape. The living was easy, but building a nest and feeding babies was an impossible task. Following the food source created by the buffalo herd might place you a hundred miles away from your nest and hungry babies within a few days. So, over evolutionary time cowbirds came up with another solution--they laid their eggs in the nests of more sedentary birds while they themselves tagged along after the buffalo. Most buffalo herds have disappeared, and some cowbirds have moved east, yet they have maintained their brood parasite strategy.

In addition to the phony foster chick scam, some birds engage in another practice that, from the perspective of humans, is particularly unsavory: siblicide. Siblicide, in which nestmates kill a brother or sister and throw its carcass out of the nest, has been reported for many types of birds, including eagles and egrets.

Cowbirds do not generally kill their nestmates. But the cowbird's behavior is not as altruistic as it appears; it is, in fact, self-serving in an unexpected way. Rebecca M. Kilner of Cambridge University and colleagues studied brown-headed cowbirds to learn why a baby cowbird typically does not kill the other baby birds, which would lead to its getting all the food its foster parents bring to the nest. As the researchers put it, nestling parasites like cowbirds "should be ruthlessly self-interested and should kill host offspring soon after hatching." The scientific challenge was to determine why a species performs in a manner seemingly contradictory to its best interest.

The researchers set up tests using eastern phoebes, a type of flycatcher, as the host species and arranged for a single cowbird egg to be in each of 20 nests. In 10 nests, they removed all eggs of the phoebe on the day the cowbird egg hatched so that the parent birds had a single baby they assumed was their own. In the other 10 nests, they removed all the hosts' eggs on the day the cowbird hatched but added two newly hatched phoebes. Adult phoebes were tending 20 nests; 10 had a single cowbird and 10 had a cowbird with two phoebe nestmates.

The results were surprising. Cowbirds with two nestmates gained weight more rapidly than those in a nest alone. The investigators filmed the nests and found that parent birds brought food to three birds about 2½ times more often than to a single cowbird. However, a cowbird in the nest with multiple birds took more than half the food brought to the nest, so it fared better than a lone cowbird getting all the food. The mystery of the cowbird's seeming altruism toward other baby birds was revealed: not engaging in siblicide turns out to be a strategy for getting more food.

Do the cowbirds always win? Nope. Nature seldom works that way. Some birds are aware that the cowbird egg does not belong in the nest. Recognizing it as a parasite, the would-be foster parents let it languish unattended or actually remove it from the nest. Yellow warblers will place new nest material over the uninvited egg and lay their own eggs on the new layer. The record is six layers of nesting material covering six cowbird eggs. The name of the game, after all, is survival of the fittest and there are many ways to play it.

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