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Brown-headed Cowbird

(Molothrus ater)
Banded May 16, 1998 - Carmel, Indiana

General Information

The Brown-headed Cowbird is one of two species of cowbirds found in North America. Both species are brood parasites that lay their eggs in the nests other birds. Formerly occurring in the central grasslands of North America, wholesale clearing of forested land has allowed the Brown-headed Cowbird to extend its range across most of North America, and to increase its population dramatically.

The male has an iridescent black body and a brown head. The female is slightly smaller than the male and uniformly gray.

Figure 1 - Male Brown-headed Cowbird

Figure 2 - Male Brown-headed Cowbird

The coffee-brown head of the male is a distinguishing field mark. The strong conical bill is adapted to its diet of insects (especially grasshoppers) and seeds and berries.

The egg of the Brown-headed Cowbird is white, bluish or greenish with a fine speckling of reddish brown. In this photo, the cowbird chick is in the process of hatching. Cowbird chicks usually hatch a day or two before the eggs of the host bird and grow rapidly, giving them a competitive head start.

Figure 3 - Cowbird Egg in Chipping Sparrow Nest

Figure 4 - Cowbird Eggs in Song Sparrow Nest

Two Brown-headed Cowbird eggs have been laid in this Song Sparrow nest. Even though a bit larger, note the similarity of the cowbird eggs to those of the Song Sparrow. Some host species recognize and reject cowbird eggs, but many host species such as the Song Sparrow do not distinguish between their own eggs and those of the cowbird.

The upper wing coverts of the adult male are black. Those of younger birds show a brownish wash.

Figure 5 - Upper Wing Coverts

Figure 6 - Primary Flight Feathers

The primary flight feathers of the adult male are black. Those of the female are more brownish gray.

Under wing coverts of the adult male are black. Those of younger birds are more brownish gray.

Figure 7 - Under Wing Coverts of Adult Male

Figure 8 - Under Tail Coverts

The under tail coverts are also black in adult males.

Tail feathers of the adult male are also black.

Figure 9 - Tail Feathers

Nesting Behavior

The Brown-headed Cowbird does not build a nest of its own. The female lays her eggs in the nests of other bird species. The host parents then raise the cowbird chick as their own. The cowbird's egg usually hatches a day or two before the host's eggs. Rapid growth allows the cowbird chick to out compete the host's chicks for food and space in the nest. The result is that the host's chicks usually perish. Female cowbirds have a long reproductive period and can lay as many as 80 eggs over a two month period.

Banding Recoveries

The Bird Banding Lab web site reports that between 1955 to 1997, a total of 834,539 Brown-headed Cowbirds were banded. Of these, 14,148 have been recovered, a recovery rate of 1.69%.

Banding studies show that Brown-headed Cowbirds are short distance migrants that winter in the southern U. S. and Mexico. They may live up to 13 years in the wild. If you should recover a banded bird, report the band number to the Bird Banding Lab by calling 1-800-327-BAND.

Conservation

Populations of Brown-headed Cowbirds are increasing at an alarming rate in many regions. Because they are brood parasites, they are a particular threat to populations of many other species of birds, especially endangered species such as Kirtland's Warbler. Arguably, reduction of Brown-headed Cowbird populations would be of benefit to many other songbird and gamebird species.

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