City of Austin -

The Brown-headed Cowbird (Molothrus ater) is a year-round native bird species in Texas that is a management issue for the Balcones Canyonlands Preserve because it is a brood parasite. This means that it lays its eggs in the nests of other birds for the "host" birds to raise, never building its own nest. It is not selective in this behavior and is known to parasitize the nests of between 100 and 225 species of birds, including the black-capped vireo and golden-cheeked warbler. However, bird species that construct open, cupped nests in shrubs in more open areas, such as the black-capped vireo, are particularly vulnerable because studies have shown cowbirds have a preference for woodland edges as opposed to areas deeper into a large, wooded area.

Their behavior today makes more sense when one understands how they lived prior to European settlement. They followed herds of buffalos in the open grasslands of the central North American plains and consumed insects stirred up by the animal's grazing and movement. When this source of food they relied on moved, they also needed to move, which didn't allow time for nest building and raising young. Thus they would lay their eggs in the nests of songbirds and migrate with the herd. The gradual change from migrating buffalo to large livestock (cattle, sheep and horses) restricted to a ranch, combined with the fragmentation of woodlands and increase in edges, and introduction of agriculture all provided suitable habitat. The result has been a significant expansion of their range and increase in their numbers, giving them access to a wider variety of species' nests to utilize. Today, biologists see these birds as one of the reasons so many other of our native species are not having the reproductive success they should.

Physically, the cowbird might be described as "average" in size between seven and nine inches long. Males have an iridescent black body and wings with the characteristic brown head and neck. Females can appear as a mottled light brown to dull grey all over and are slightly smaller than males. They both have a short, conical, grey bill, black eyes, long, pointed wings, and slightly rounded tails. One of their most recognizable calls is a bubbly, relatively high-pitched song. They can be seen often in mixed-species flocks with other blackbirds, such as the Common Grackle. When on the ground they typically walk, foraging for seeds and insects.

Despite the complete absence of parenting behavior, these birds are quite prolific. A single female can lay up to 40 eggs in one year in the nests of other host birds. She will watch for the comings and goings of other, usually smaller, birds or will attempt to flush them out in order to find a nest. If the host bird has begun to lay eggs, then the female cowbird will typically lay one or two of her own. Cowbird eggs are whitish with brown speckles. Sometimes, she may kick out or eat the eggs of the host bird. Cowbird chicks often hatch one to two days before the host's young and grow rapidly. Young cowbirds have not been documented intentionally pushing out their smaller nest-mates; they simply out-compete them for food.

Thus, management is required to reduce the number of cowbirds in the hopes the ecosystem will return to something closer to a balanced system. Texas Parks and Wildlife recommends several strategies in a brochure it produces entitled "Controlling Brown-headed Cowbirds". One such recommendation is inherent in the acquisition that has occurred of contiguous and "whole" tracts for the golden-cheeked warbler; it is to maintain large areas of woodlands and minimize the amount of edge in proportion to the interior. Thus, conservation of a large wooded area closer to a square or circle in shape would be better than the conservation of a narrow and long wooded canyon. It is important that conservation of woodlands occurs on a landscape level since these birds can travel up to 30 miles between foraging and egg-laying areas.

Another recommended strategy utilized by the City's BCP program is trapping, which has been shown to reduce cowbird parasitism in localized areas. Staff with the assistance of volunteers operates several strategically placed cowbird traps during the breeding season of the two endangered bird species. The design of the traps and protocol for operation is set up to avoid harm to non-target species and the humane treatment of trapped cowbirds prior to euthanasia. As is stated in the TPWD brochure, "per Parks and Wildlife Code, Section 64.002 (b), brown-headed cowbirds are included among the small group of eight non-protected bird species that "may be killed at any time and their nests or eggs may be destroyed".

TPWD has been given the authority by the U.S. Fish & Wildlife Service to conduct a pilot program to train private land managers in trapping methods. Once trained, individuals are "covered" under the state organization's permit and may begin trapping on their own land and asked to report their data.