Eastern Phoebe nest with Brown-headed Cowbird egg
After reading a recent blog entry about brood parasitism in cliff swallows, a number of readers wanted to know more about the well-known sort of brood parasitism practiced by cowbirds.
If a vote were taken tomorrow to find the most popular bird in the country, it’s doubtful that the cowbird would win. That’s because the cowbird has the nasty habit of laying its eggs in other birds’ nests.
It all started back in the days when American Bison ruled the Great Plains. At that time a rather nondescript grayish or black bird followed the herds around, feeding on seeds in the abundant supply of buffalo excrement. Settlers on the plains came to calling these animals buffalo birds.
But since the birds depended on wandering herds for food, they needed to wander as well if they wanted to survive.
The problem, of course, is that wandering birds can’t tend their nests. So the buffalo birds decided to leave their young in the care of other birds, an arrangement that seemed to work, at least from the buffalo birds’ perspective.
Then, during the 1800s, the prairies and buffalo disappeared, replaced by pasture and cattle. But the birds remained and started keeping company with cows instead of buffalo, eating insects in the grass, ticks on the livestock, and seeds and grain. The buffalo bird eventually became known as the cowbird.
Today there are two native cowbird species in North America, the Bronzed Cowbird of the Southwest and the Brown-headed Cowbird common in most of the United States and Canada. Both species still lay their eggs in the nests of other birds, which is to say that both maintain the parasitic tradition of their ancestors, much to the dismay of bird lovers and conservationists.
The main reason people find the cowbirds’ behavior objectionable is that it threatens biodiversity.
Cowbirds as a whole lay their eggs in the nests of more than 200 other species of birds. And in most cases, because these birds tend to be smaller species, the young cowbirds come to dominate the nests, pushing out the other young or hoarding the food. The result is that the two cowbird species thrive at the expense of hundreds of others.