Brown-headed Cowbird (Molothrus ater) With Permission LSU AgCenter
The brown-headed cowbird, or simply cowbird as it is often called, is approximately the size of a house sparrow (6-8 inches in length). The entire head, neck and upper breast of males are buffy brown while the rest of the plumage is a metallic green-glossed black. Females are a dark buffy brown above and smokey gray below with a throat patch that is white or pale gray.
The brown-headed cowbird was once a bison-following species of the Great Plains that moved eastward throughout the 1800s as forests were cleared. It is now common across North America in both breeding and wintering populations.
Cowbirds have the dubious distinction of being a nest parasite, with females laying their eggs in nests constructed by other avian species. It has been theorized that this trait evolved from centuries of following bison herds to feed on the insects flushed from the ground as these large animals traveled in their seasonal migrations. Female cowbirds were never able or willing to invest in the time to build their own nests and instead found the nests of other species in which to lay their eggs. As bison herds disappeared and cowbirds made their way to the eastern United States, their habit of nest parasitism remained as part of their reproductive cycle. Their eastward movement was encouraged by increased forest openings from European settlers.
More than 200 different avian species are known to have nests parasitized by the brown-headed cowbird. Once a nest is chosen, a female cowbird will lay one of her eggs, which is gray with brown markings. Her eggs are usually distinguishable from the smaller eggs of the nest-building female. Some parasitized species simply abandon their nest or build a new nest on top of the old one.
Other species with large bills, such as bluejays or robins, will remove the foreign egg from their nest. More than 150 species, however, are known to serve as suitable foster parents, brooding and feeding all young that are hatched. Cowbirds develop rapidly and hatch in only 10 to 13 days, which, on average, is about one to six days sooner than the others in the nest. The young cowbird hatchlings begin a voracious demand for food, and females will often forgo caring for their own young to meet these demands. This type of “chance” egg-laying leads to only 3 percent of cowbirds eggs hatching and reaching adulthood. A low survival rate is compensated by females having the capacity to lay 40 or more eggs each nesting season.
Control Measures: Brown-headed cowbirds are considered a nuisance species in Louisiana and can be taken year-round during legal shooting hours if they are depredating or about to depredate agricultural crops or wildlife. They are grouped along with blackbirds, crows and grackles in this regard.
Cowbirds favor forest edges, and forest fragmentation has led to a great increase in these edge habitats and a subsequent reduction in forest interior habitats that cowbirds will not penetrate. Forest management efforts aimed at increasing the amount of habitat in the forest interior is a way of protecting many of our threatened songbirds in North America from depredation by brown-headed cowbirds.