The Brown-Headed Cowbird

by Kenneth A. Kuiper, Biologist
Natural Resources Conservation Service (NRCS)
Salina, Kansas

Brown-headed cowbirds, commonly seen catching insects within a few feet of livestock on Kansas ranches, go mostly unnoticed. One young cowbird was noticed by a Kansas citizen this summer in her backyard in Salina. The questions from this encounter were, "Why was a cardinal feeding a black bird? Where was the fledgling cardinal that should have been cared for and fed by the parent?"

The biologist’s response is, "You probably observed a sinister example of brood parasitism. It sounds like you observed an event in the life cycle of the native brown-headed cowbird."

Brown-headed cowbirds, Molothrus ater, are brood parasites. These birds, according to a Cornell University publication, have completely abandoned nest building, incubating eggs, and caring for hatchlings. A female can deposit as many as 40 eggs per year in the nests of other birds and these birds raise cowbird young.

More than 100 other bird species are known to provide host nests for cowbird eggs. The female cowbird finds the nests of other birds by observation. They also crash noisily through shrubbery by wing flapping to flush out potential victims. The opportunity to lay an egg commonly takes place at dawn in a nest with eggs that are smaller than the cowbird egg.

Are cowbirds a problem? Do cowbirds negatively affect native song birds? Can anything be done to control cowbirds?

A recent study by Jack F. Cully Jr. and William E. Jensen, Kansas Cooperative Wildlife Research Unit, Division of Biology, Kansas State University, shows that high cowbird density is an important factor where cowbirds are successful in parasitizing host nests. This study took place in the Flint Hills of eastern Kansas and Oklahoma.

In Texas, there is an active cowbird control program in a diagonal geographic area across the state. The issue there is loss of endangered black-capped vireo, Vireo atricapillus, habitat. According to Texas Natural Resources Conservation Service (NRCS) personnel, ranchers are willing and positive about participating in habitat management and direct control measures where cowbird density is a problem.

Black-capped vireo habitat loss in Texas is also attributed to urbanization. In Austin, Texas, a bond election approved $22 million for the purchase of Resolution Trust Corporation lands to preserve vireo habitat.

In pre-colonial North America, brown-headed cowbirds followed bison herds. The life history strategy of laying eggs in other birds’ nests was a survival mechanism. Bison grazing provided the insects upon which adult cowbirds fed, but the constant moving of the bison herds would create a distance problem between the adult food source and incubating eggs in the nest. Cowbirds solved the distance problem by finding surrogate parents for their young.

The cowbird's range expanded because of the clearing of forested landscapes and the introduction of large domestic mammals, according to Cornell University. Horses, cattle, and other animals provide a stable, relatively immobile, supply of insects for adult cowbird food. This allows more time for parasitizing native bird nests.

In Texas, cowbird control can include large portable traps placed on ranches in locations with high cowbird density like livestock watering locations. Traps are mounted on trailers so they are easily relocated as livestock grazing areas change. These traps are usually set, monitored, and managed by an organized research project sponsor and they are always part of the U.S. Fish and Wildlife Service Black-Capped Vireo Recovery Plan where this species is endangered.

This article is meant to provide a few facts, a bit of information, and awareness about brood parasitism. It is not suggesting initiation of trapping or control schemes in Kansas. Control measures must be implemented with the Kansas Department of Wildlife and Parks’ approval after an assessment of regulatory implications.

Two existing practices found in the Kansas NRCS’s electronic Field Office Technical Guide (eFOTG) have the potential to address wildlife species for the purpose of conserving biodiversity. It is logical to suggest non-regulatory practices like Restoration and Management of Rare or Declining Habitats or Upland Wildlife Habitat Management could improve habitat for native songbirds and reduce brown-headed cowbird parasitism.

For more information, go to your local U. S. Department of Agriculture’s Service Center and talk to the NRCS or conservation district staff.

For more information about NRCS programs, visit the Kansas NRCS Web site at www.ks.nrcs.usda.gov.

This article is also available in Microsoft Word format.

The Brown-Headed Cowbird (DOC; 42 KB)

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Last Modified: 08/14/2008