Hear a story about the brown-headed cowbird.
Hear a brown-headed cowbird.

adapted from The Birds of Texas
by John L. Tveten

Cowbirds are related to blackbirds, grackles, orioles, meadowlarks and bobolink -- all of which are known at "icterids" (from the Subfamily Icterinae). This heterogeneous group of nearly 100 species ranges throughout the New World, from Alaska to Tierra del Fuego, with most of its members occurring in the tropics and subtropics. Two cowbird species inhabit Texas -- the brown-headed cowbird and the bronzed cowbird. The brown-headed cowbird ranges across the entire state, while the bronzed cowbird inhabits South Texas, from Eagle Pass through the Rio Grande Valley.

The brown-headed cowbird is a chunky little bird scarcely seven inches long with a short, conical, sparrowlike bill. The glossy black male wears a chocolate brown hood; the female is entirely dusky brownish-gray. It is a gregarious bird whose calls resemble a harsh rattle and squeaky whistle.

Brown-headed cowbirds once lived primarily on the Great Plains, where they followed herds of American bison. With the clearing of the forests, they expanded both eastward and westward, and cowbirds now occupy most of the continent south of the Arctic, from Canada through northern Mexico. They feed primarily on seeds and grains, and roam year-round through open woodlands, farms, ranches and suburbs. In winter they join mixed flocks of other icterids, among which they can be recognized by their small size and their habit of holding their tails lifted high as they walk about in search of food.

It is their breeding habits that have brought brown-headed cowbirds into disfavor, for they are brood parasites. Rather than building their own nests and caring for their own young, females simply lay their eggs in the nests of other species. As a result, theirs is a vagabond life style, for they have no household chores to tie them down.

Brood parasitism often causes other birds' nests to fail and has led to the decline of many bird species. Cowbird eggs have been found in the nests of at least 220 different species, but not all successfully raised the chicks. Because cowbird eggs have a short incubation period of only 10 to 13 days, they usually hatch a day or two before the eggs of other species. In addition, the young cowbirds are larger and tend to grow more rapidly than most of their nest mates. They therefore receive most of the food and may even crowd the others from the nest.

Brown-headed cowbirds parasitize a wide range of species but prey most heavily on flycatchers, vireos, warblers and sparrows. In the Texas hill country, cowbirds have contributed to the decline of endangered black-capped vireos and golden-cheeked warblers. Cowbird eradication and removal programs have been implemented in some areas of the hill country and in other regions of the U.S. to help slow the decline of endangered bird species.

More Facts... The genus name for cowbird - Molothrus - is apparently a misspelling of the Greek "molobros," meaning "a parasite or greedy vagabond." Cowbirds are also known as cow-pen birds since they congregate around cattle. The female lays only one egg in a nest. About 1/3 of all parasitized nests hold more than one cowbird egg, apparently from different females. Only 3 percent of cowbird eggs result in adults, but the average cowbird lays 40 eggs per year for two years. At this rate, a female generate 2.4 adults. Each pair replaces itself with an average of 1.2 pairs, thereby doubling the cowbird population in 8 years.

More Links Hear an interview on cowbirds and their impact on black-capped vireos on the Kerr Wildlife Management Area. USGS website with photos and scientific data on the brown-headed cowbird. More information on cowbirds and brood parasitism.