Brown-headed Cowbird, Molothrus ater
The Brown-headed Cowbird is a brood parasite, meaning it lays its eggs in the nests of other species and builds no nest of its own. This strategy was very effective when this bird was limited to the short-grass plains and followed herds of buffalo eating the insects they would stir up by their movements. A cowbird couldn't afford to stay in one location long enough to nest and raise its own young.
Now, however, the Brown-headed Cowbird range has expanded across most of North America as European settlement opened forests for agriculture and pastures. It now parasitizes the nests of many new species that had no previous exposure to nest parasitism. Most native birds cannot distinguish their own eggs and nestlings from that of a cowbird and will raise the cowbird nestling at the expense of their own young.
Outside of breeding season, the Brown-headed Cowbird forages in large flocks with other blackbirds primarily in agricultural areas in the southern portion of the breeding range. This species was not found in Tennessee until the late 1800s and is now a year round resident.
Description: The Brown-headed Cowbird is a medium-sized songbird with a stout pointed bill. The male is shiny black with a brown head and neck; the female is a dull gray-brown all over. The juvenile (June-September) is similar to the female, but has brown streaking on the breast.
Weight: 1.5 oz
Voice: The song is a variety of chatters, high pitched liquid notes, and a frequent two-note tee deewhistle given in flight.
- Female and juvenile Brown-headed Cowbirds resemble sparrows, but are larger, completely dull gray-brown, and have unstreaked backs.
- Female Red-winged Blackbirds are more heavily streaked.
Habitat: Breeds in areas with grassland and low or scattered trees, such as woodland edges, brushy thickets, fields, prairies, pastures, orchards, and residential areas. Fragmentation of forests has greatly increased potential breeding habitat.
Diet: Brown-headed Cowbirds forage on the ground on insects and seeds, often in association with cows or horses. They will frequently feed on the ground below backyard feeders.
Nesting and reproduction: Brown-headed Cowbirds do not defend a territory and males and females will mate with several individuals within a season. Females lay eggs in Tennessee from mid-April through mid-June.
Clutch Size: A female can lay as many as 40 eggs in the nests of different species within one breeding season, usually one egg per nest. Many of these eggs are not successful.
Incubation: The host incubates the eggs, which hatch in 10 to 12 days (shorter than most songbirds)
Fledging: Cowbird young fledge in 10 to 11 days.
Status in Tennessee: The Brown-headed Cowbird is a common permanent resident in the state. The first reports of nesting of the species in Tennessee were from Roane County in 1886, Reelfoot Lake in 1895, Nashville in 1918, Shelby County in 1921, and East Tennessee in 1932. The population is declining in the state for unknown reasons.
- Brown-headed Cowbirds are known to have parasitized 44 different species in Tennessee (as of 1991), and 220 species rangewide.
- Female cowbirds have no territories and wander widely. They may lay as many as 40 eggs per season!
- While some female cowbirds will lay their eggs in the nests of a number of different species, most females specialize on one particular host species.
- Recent research has found that female cowbirds will monitor nests where they have laid eggs. If their egg(s) is removed, they may destroy the host's eggs! Scientists originally thought that there was no parental investment after laying the eggs. Watch NOVA video about this study.
Obsolete English Names: cowbird, cow-pen-bird, buffalo-bird
Best places to see in Tennessee: Brown-headed cowbirds are common in open country and agricultural lands and rare in heavily forested areas. In winter, they are found in flocks with other blackbirds primarily in agricultural areas.
Lowther, P. E. 1993. Brown-headed Cowbird (Molothrus ater), The Birds of North America, No. 47 (A. Poole and F. Gill, eds.). The Academy of Natural Sciences, Philadelphia, PA, and The American Ornithologists' Union, Washington, D.C.
Nicholson, C. P. 1997. Atlas of Breeding Birds of Tennessee. Univ. of TN Press, Knoxville.
Sibley, D. A. 2000. The Sibley Guide to Birds. A. A. Knopf, New York, NY.