ARE NOT AS NICE AS THEY SEEM
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.
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.
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