Brown-headed Cowbird    Molothrus ater

Migratory Status: Short-distance migrant

PIF Population Estimate: 56 000 000

Percent of western hemisphere population breeding in boreal forest: <25%

BAM Effective Detection Radius (m) : 73.87

PIF Maximum Detection Distance (m) : 125

Canadian BBS Population trend: -2.2 (-3.0 to -1.4)

Life History

The historic range of the Brown-headed Cowbird was once limited primarily to the short-grass prairies of North America. Known as “buffalo birds”, they would follow vast herds of bison (Bison bison) and feed on insects flushed by the movement of feeding animals. Because the herds were highly mobile, the cowbirds adapted a unique breeding strategy, whereby females deposit their eggs in the nests of territorial songbird species, leaving these hosts to feed and raise the cowbird young.

In recent decades, with the further expansion of agricultural and other anthropogenic land clearing, populations of the Brown-headed Cowbird have expanded into many new areas. With this expansion, songbirds with no previous exposure to this form of nest parasitism have been greatly impacted. Often the eggs or nestlings of resident species are denied attention from their parents or ejected from the nest by the larger and more aggressive cowbird chick. As female cowbirds can lay up to 40 eggs in a single breeding season, the impacts on host populations can be significant.

Many species with endangered or threatened status designations in the United States have been severely impacted by high rates of nest parasitism by cowbirds. As a result, many agencies have adopted aggressive cowbird control programs as a means of reducing rates of parasitism in sensitive host populations.

The Brown-headed Cowbird’s breeding range currently extends over most of the North American continent, from southeastern Alaska and the southern Mackenzie District to northern Mexico, and from southern Newfoundland to central Florida. Birds in the southern and eastern United States are considered resident and those that travel south into Baja and central Mexico, and into southern Florida are considered winter migrants.

Little of the forested boreal forest provides habitat for this species and as a result it has yet to establish a strong presence here. Only in areas which have undergone recent agricultural expansion and human settlement, such as Alberta, and central Saskatchewan and Ontario, has the species expanded northward, though the impact of this expansion has not been adequately assessed.

Habitat preferences are driven largely by the selection of habitat by host species, though rates of parasitism are greatest in ecozones between forest and open grassland. The cowbird forages primarily on the ground, eating primarily insects and seeds. The species’ vocal array is quite complex, though the most frequent vocalization heard on the breeding grounds is a liquid sounding gurgle, often paraphrased as “bubble, bubble, zee”.

Reference(s)

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's Union, Washington, D.C.