Brown-Headed Cowbird

Molothrus ater

Cowbirds are well known as nest, or brood parasites. Some theorists believe that Brown-Headed Cowbirds developed this practice when they followed the great bison herds as they migrated and roamed freely from present day Mexico to Canada. To keep up with the migration, these birds would deposit their eggs in other bird’s nests, depending on their victims to raise the cowbird’s young. Cowbird chicks develop much faster than that of their hosts, so the host’s offspring rarely survive. Some host species learned to abandon an infected egg clutch and start a fresh one on top of the one with cowbird eggs. Other species have not done so well, resulting in populations declines.

Modern science recognises four subspecies of Brown-Headed Cowbird. M. a. artemisiae breeds in the interior of western Canada. M. a. obscurus breeds in coastal Alaska. M. a. ater breeds in southeast Canada and eastern and central USA. M. a. californicus breeds in southern California and northern Baja California (Mexico).

Brown-Headed Cowbirds range over most of the North American continent, migrating north as far as southern and western Canada in summer. Many of these birds remain year-round on the west coast, southwestern USA, northern Mexico, Texas and through the lower Mississippi Valley east to New England and southeastern USA to northern Florida. South Florida and southern Mexico normally see them only in winter.

It is easy to dislike cowbirds if we apply human values to their practices. When I’ve met them during my travels in the western USA, I’ve tried to remember than nature can be harsh, and my value system is irrelevant in the larger scheme of the natural world.