Brown-headed Cowbird
“Pteyahpa” is the name given by the Lakota people to the species we know as the brown-headed cowbird. Formerly occupying the short-grass prairies, these ”brood parasites” have spread across North America. Every spring a female can lay a dozen of her eggs in other species’ nests. If their egg is hatched and the host parents regard it as one of their own, it will be fed – even at the detriment of the host’s nestlings.
Although some species are able to recognize the alien egg and remove it from their nest, many others don’t see the threat. As the cowbird chick grows larger, it may force the other hatchlings out or dominate the competitive feeding by the host parents. Species that are already endangered by habitat loss, such as Kirkland’s warbler in Michigan, face the additional formidable challenge of reduced success in breeding.
In Carver County and other Minnesota environments, brown-headed cowbirds are early spring migrants arriving in March and April. They may form single-sex groups and then pair up and travel throughout our woods, fields and yards searching for the nests of unsuspecting hosts.
About seven and a half inches long, males have shiny black bodies and are distinguished from other blackbirds by their chestnut-brown heads. Females are slightly smaller and are light brown or grayish brown without distinguishing plumage markings. Both sexes have thick, cone-shaped bills. In July and August, before they migrate south, the juvenile brown-headed cowbirds may be seen at seed feeders and appear like larger versions of young house finches.
Away from our feeders, cowbirds will forage on the ground for seeds and insects, often at the edges of woods or around livestock in open fields. They are often seen flocking with common grackles and red-winged blackbirds. In preparation for migration, cowbirds form progressively larger groups. In their non-breeding territory in southern states, the flocks can contain hundreds or thousands of blackbirds and starlings.
Brown-headed cowbirds evolved in the presence of large herds of bison, feeding off the disturbed insects and grass seeds. The continual nomadic nature of the herd prevented the cowbirds from building nests. The scientific name Molothrus ater is based on the Latin word for vagabond. The species’ evolved behavior is an example of the battle for survival that exists in the natural world.
Female cowbirds will quietly surveil an area, watching for nesting behaviors of other birds. Alternatively, they also can be obvious in their presence so that nesting birds will try to drive them away. In that case, the cowbird will have confirmed the location of the guarded nest, then sneak in later and deposit a single egg.
Their offspring grow quickly and dominate, kill or out-compete the other hatchlings. As cowbirds have spread their distribution across our continent, more than 140 species have succumbed to this parasitic behavior.
Some species, including the American robin and cedar waxwing can recognize the intruding egg and remove it from their nest. Other species may build another nest layer on top of the eggs, so they aren’t properly incubated. Abandoning the nest is also a method of avoiding the problem.
Conservation efforts have been successful in reducing the impact of cowbirds on specific species by removing the cowbirds and their eggs. However, short-term success in challenging natural processes may not always have the ultimate intended effect. In our experiences with these Carver County birds, we should appreciate the opportunity to see this dynamic drama play out each summer.
For more information:
The Sibley Guide to Birds, David Allen Sibley
Birds of Minnesota and Wisconsin, Janssen, Tessen, and Kennedy
Birds of Minnesota, Robert B. Janssen
The Audubon Society’s Encyclopedia of North American Birds, John K. Terres
Birds of the World, Cornell Lab of Ornithology
Online: All About Birds, Cornell Lab of Ornithology LINK
Online: Minnesota Breeding Bird Atlas
