Brown-headed Cowbird (Molothrus ater)
Prior to European settlement, the brown headed cowbird, once called the "buffalo bird," was common in the open plains. Cowbirds followed the vast herds of American bison and then cattle, eating the insects that were stirred up by and swarmed around the hoofs of the grazing herds.
Unable to move with the wandering herds while maintaining a nest, these birds developed an unusual behavior; they began to lay their eggs in the nests of other birds, even removing removing an egg or two to make room. The cowbird chicks, which hatch earlier than most songbirds, are larger and more aggressive and will out-compete their nest mates for food. This added competition reduces the number of non-cowbird young that fledge. Cowbirds are the "tricksters of the bird world." Nest parasitism by cowbirds is a major factor in declining migratory songbird populations in much of the United States.
There may have been a limited population of brown headed cowbirds in Michigan during pre logging days, but only in the southern Lower Peninsula. However, the clearing of forests allowed the cowbird to expand its range into areas also occupied by other species, including the Kirtland's warbler. As land in Michigan was opened up during logging and agricultural development, cowbirds moved into the new areas also occupied by other species, including the Kirtland's warbler. The Kirtland's warbler was an extremely vulnerable host and soon the egg laying activity of the cowbirds began to impact the Kirtland's warbler population.
The spread of cowbirds into Kirtland's warbler habitat had a major impact on nest success. Prior to cowbird control, as many as 69 percent of Kirtland's warbler nests were parasitized by cowbirds. The U. S. Fish and Wildlife Service has used trapping as a means of reducing the cowbird population in Kirtland's warbler nesting areas. To improve nest success some alternative controls may be used, such as reducing amount of forest edges and feeding areas. Unfortunately, fragmentation of forest habitats by current land use practices will continue to keep the cowbird in Michigan.