by Whit Gibbons

August 29, 2004

Like all birds, brown-headed cowbirds operate on a simple principle: survival of the fittest. In the cowbirds' case, this includes being "brood parasites," a nasty-sounding name for a nasty behavior, at least from other birds' perspectives. Cowbirds lay their eggs in the nests of other birds and let the foster parents raise baby cowbirds along with their own. At least 100 species of birds are known victims of the cowbird's sneaky behavior.

Cowbirds, native to areas of the United States where buffalo once roamed, made a living by eating insects stirred up by the big herds as they moved across the landscape. The living was pretty good, but when your traveling companions are ever-moving buffalos, building a nest and feeding babies becomes an impossible task. Following the food source created by the buffalo herd might place you a hundred miles away from your nest and young within a few days. So, cowbirds 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, and continue to be brood parasites.

But the phony foster child scam does not always end when a female cowbird deposits an egg in the nest of an unwitting sparrow or red-winged blackbird. The adult cowbird may actually eat an egg or two of the host bird. Babies of the European cuckoo, also a notorious brood parasite, go a step further and kill the other babies when they hatch. But baby cowbirds usually do not kill their nest mates.

One challenge for scientists is to devise ways to understand behavioral ecology and evolution, especially when a species performs in a manner seemingly contradictory to what would be in its best interest. Why doesn't a baby cowbird kill the other babies so it will get all the food the parents bring to the nest? Contrary to the childhood dictum that "little birds in their nest agree," individuals of a species look out for themselves without regard for the welfare of others. Some birds routinely kill off their siblings. So why would baby cowbirds play nice in the nest?

Rebecca M. Kilner of Cambridge University and colleagues studied brown-headed cowbirds in search of an explanation for why a baby cowbird will typically let babies of the host species live. As the researchers put it, nestling parasites like cowbirds "should be ruthlessly self-interested and should kill host offspring soon after hatching."

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. In other words, adult phoebes were tending 20 nests, 10 with a single cowbird and 10 with a cowbird and two phoebe nest mates.

The results were surprising. Cowbirds with two nest mates gained weight more rapidly than those in a nest alone. By filming the nests the investigators found that on average parent birds brought food to three birds about 2½ times more often than to a single cowbird. Since a cowbird in the nest with multiple birds took more than half the food brought to the nest, it fared better than a lone cowbird getting all the food. So a cowbird's seeming altruism toward other babies is simply a strategy to get more food.

Are you still rooting for the underdog, or the underbird as it were? Okay, cowbirds do not always win. Some birds recognize the cowbird egg as a parasite and let it languish unattended; they may even remove it from the nest. According to the Cornell Laboratory of Ornithology some smaller birds, such as yellow warblers, 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.

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