I was catching up on some breeding bird atlas work last weekend when I was alerted to the sound of a begging baby bird. I soon located the young one and watched as a blue-gray gnatcatcher flew in and fed it a caterpillar.
Unfortunately the baby was a brown-headed cowbird, a species about twice as long and five times as heavy as a gnatcatcher.
Was this a case of mistaken identity? Actually, no. The gnatcatcher had incubated the cowbird egg and raised the chick as its own, to the detriment of its own progeny. To the gnatcatcher, the cowbird was its true chick.
Brown-headed cowbirds are the only species of North American bird that does not care for its own offspring. Females lay their eggs in other species’ nests, often a much smaller species.
The larger and more aggressive cowbird chick tempts the foster parents into feeding it more and has the ability to actually evict its smaller nest mates over the sides of the nest. I have seen baby cowbirds being fed by other species many times, and the cowbird is the only baby bird in sight. The other chicks just never survive. Only one cowbird egg per nest, too, so the cowbird chicks do not have to compete with other cowbirds.
It is thought that cowbird brood parasitism is playing a part in the decline of some neotropical migrants that breed in our area. In some other parts of the country, cowbird eradication programs have been approved to protect endangered species. The programs have proven to be successful but are also somewhat controversial. The brown-headed cowbird is a native species acting in the way they have evolved.
Cowbirds like large animal herds, and evolved following the herds of North American bison. Since these herds were constantly moving, it was not practical for the cowbird to have to stay in one place for a couple of months to raise young. Thus this unique reproductive strategy was developed.
Cowbirds can form flocks of thousands of birds in the winter. Consider that each single cowbird represents a failed nesting of another species of bird.