Brown-headed Cowbird

Brown-headed Cowbird
In the open grasslands of precolonial North America, the Brown-headed Cowbird's habit of depositing its eggs in the nests of other species freed it to follow the peregrinations of the bison herds. The bisons kicked up insects upon which the cowbirds fed. Still associated to some extent with large mammals, such as horses, cattle, and sheep, the Brown-headed Cowbird has now greatly expanded its range throughout the modern landscape from coast to coast.

Brown-headed Cowbirds are brood parasites, that is, they have completely abandoned the tasks of building nests, incubating eggs, and caring for hatchlings. Instead, each female deposits as many as 40 eggs per year in nests that belong to other bird species. More than 100 other species have provided host nests for cowbird eggs. The female cowbird finds these nests by watching patiently from an observation post where she can look down upon grassland species, by observing the nests of tree-nesting species while she walks quietly on the forest floor, or by crashing noisily through shrubbery with flapping wings to flush out potential victims. She typically chooses a nest with eggs smaller than her own and lays a single egg quickly at dawn once the host has also started laying eggs. Unlike the parasitic European cuckoos, Brown-headed Cowbirds do not evict their nest-mates, although the female may remove and sometimes eat eggs from the host nest. Instead, cowbird nestlings typically out-compete their smaller nest mates.

The cowbird's range expanded soon after many forested landscapes were cleared and large domesticated mammals were introduced. These events brought the cowbird into contact with naive populations of potential hosts, many of whom have not yet had time to evolve strategies for dealing with brood parasitism. Most species will abandon their nest if the cowbird egg is laid first. Some species recognize and reject cowbird eggs, while others appear unable to distinguish either egg or nestling from their own legitimate offspring. When the egg is recognized but the host species is too small to remove the egg, the nest may be abandoned, or new nest material may be placed over the cowbird egg, insulating it from being incubated. Nests of Yellow Warblers, a frequent cowbird victim, have been found with up to six stories, as multiple Brown-headed Cowbird eggs were recognized and covered.
Brown-headed Cowbird Range Map

The most common hosts of Brown-headed Cowbirds are Yellow Warblers, Song Sparrows, Red-eyed Vireos, Chipping Sparrows, Eastern Phoebes, Eastern Towhees, Ovenbirds, and Common Yellowthroats. The Red-winged Blackbird is an important host because of its sheer abundance, although the actual percentage of nests parasitized is relatively low. Although the Brown-headed Cowbird's individual host's nesting success is adversely affected, most species have not suffered population declines as a result of brood parasitism. Exceptions, however, include Black-capped Vireos, Least Bell's Vireos, and the endangered Kirtland's Warbler, a species whose very existence may depend on the continued control of Brown-headed Cowbird numbers within its limited range. In addition, brood parasitism by Brown-headed Cowbirds might contribute to the reduced success of some species in fragmented forest environments.

Brown-headed Cowbirds are partially migratory, abandoning northern parts of its range and wintering most abundantly in the southeastern United States, Southern California, and Arizona. During migratory flights, cowbirds associate in flocks with Red-winged Blackbirds, Common Grackles, and Rusty Blackbirds, less commonly with American Robins and Eastern Meadowlarks in the East and with Brewer's Blackbirds and Yellow-headed Blackbirds in the West. Like other blackbirds, Brown-headed Cowbirds shift their diet at the end of summer away from insects to grains primarily in the fall and winter, foraging on the ground by walking, tail held nearly upright and wings drooping

In many parts of their range, transient Brown-headed Cowbirds that will move farther north appear in early spring, followed by resident males, and about ten days later, by resident females. The last to arrive are immature males and females. Males outnumber females by a three to two ratio in adults, but in spite of this, cowbirds tend to be monogamous.

Males select a "singing tree" from which to sing the typical song beginning with two liquid whistles and ending in a rapid glissando sometimes written as bub-blow com seee. This song is almost always accompanied by a "bow" or "tip-over" display toward either a female or a rival male. Another commonly heard vocalization is given just before or during flight. This "flight whistle" consists of two or three clear, high-pitched notes.

Description: Brown-headed Cowbirds are relatively small blackbirds with short, conical bills; rather long, pointed wings; and slightly rounded tails. Males are uniformly glossy greenish black with a brown head and neck. The eyes, bill, legs, and feet are black. Females are brownish gray above, with a faint greenish gloss. The wings and tail are more dusky brownish with pale feather edgings. The chin and throat are paler, sometimes almost white. Juveniles look much like females, but they are striped below.

Two other cowbirds range into the United States from the tropics. The Bronzed Cowbird (Molothrus aeneus), which occurs in southern Texas and Arizona, has a larger bill and red eyes. Male Bronzed Cowbirds have a hunchbacked look and bluish black wings and tails. In Texas, females are uniformly black, but they are duller than the males; the southwestern form of females are dark gray. The Shiny Cowbird (M. bonariensis), a South American species, recently expanded its range across the Caribbean to Florida in an expansion similar to that of the Brown-headed Cowbird. Shiny Cowbird males are glossy black with purple or purplish blue on the head, neck, breast, and upper back and greenish blue on the wings. The females are gray brown, somewhat darker above than below, with whitish eyebrows and throats.