From The New York Times
Sometimes the scientists who study animal behavior solve puzzles and other times they uncover new ones. The war between mockingbirds and cowbirds is a case in point.
Cowbirds are brood parasites, meaning they lay their eggs in the nests of other bird species, thus unloading the messy and demanding business of chick-rearing. They also peck holes in the eggs of the host birds, destroying as many as they can.
Mockingbirds are a favorite target of this plan, and it seems to make perfect sense for them to viciously attack cowbirds when they catch them in the nest.
But when Ros Gloag, then a doctoral student at Oxford, and her colleagues in Argentina looked closely at the war between chalk-browed mockingbirds and shiny cowbirds, they found something unexpected, as they reported in the November issue of Animal Behaviour.
They stationed small video cameras near the nests of 40 pairs of chalk-browed mockingbirds. Over two breeding seasons they recorded more than 200 attacks on intruding cowbirds.
They were surprised to find that these attacks, which their videos show to be quite vicious, did not stop the cowbirds from laying eggs. The cowbirds would hunker down and let the much large mockingbirds deliver hammer blows to the head, but in matter of seconds they would lay an egg and flee.
How could such a failed strategy persist in evolution?
The answer, according to Dr. Gloag, now a postdoctoral researcher at Australian National University, was that the attacks did prevent the cowbirds from carrying out the second prong of their plan, destroying the mockingbird eggs already in the nest.
So the mockingbirds managed to save some of their offspring. They still end up raising unwanted baby cowbird guests. But, said Dr. Gloag, once the eggs hatch, the larger mockingbird chicks compete very well with the smaller cowbirds.