A brown-headed cowbird chick is the first to hatch in this eastern phoebe nest. Cowbirds hatch a few days earlier and are often bigger and more aggressive than the nesting species. Photographs by Dennis Oehmke.

Brown-headed cowbirds are a parasitic species, meaning they lay their eggs in other birds' nests! Their eggs hatch a day or two earlier than most birds, and the chicks are larger and more aggressive. They often out-compete the host babies for food.

Migratory songbirds like warblers, phoebes and others are especially at risk. Because cowbirds frequent forest edges, songbirds can be safe in the interior of large unbroken forests. Unfortunately, few large tracts remain today. Road building and development by people creates avenues into forest interiors for cowbirds, blue jays, raccoons and others edge-dwelling species that prey on songbirds.

Cowbirds can even disguise their eggs by mimicking the look and color of the host bird's eggs!

They have found that this arrangement works to their advantage. Other birds raise their young, and they only need to find food for themselves.

In "The Birds of Illinois," H. David Bohlen lists 61 species of birds affected by brown headed cowbirds. Some of the more interesting ones include warblers, vireos and tanagers. These birds are already faced with the loss of habitat, both in North America and in their wintering homes in Central and South America. Couple that, with the dangers of a grueling migration, and now, cowbird parasitism.