Wild Side: Cowbirds

Nobody likes a parasite.

The brown-headed cowbird is a respectable native bird and a smaller relative of the familiar red-winged blackbird and common grackle. —Alan D. Wilson

While birds rank among the most charismatic of wildlife, there are a few avian species that arouse the ire of birders and naturalists. Often these species are introduced ones. The European starling, for example, was introduced from Europe to North America, and is now roundly loathed for its vigorous competition against certain native birds for nesting sites.

A bit of an oddity, then, is the brown-headed cowbird, a respectable native bird and a smaller relative of the familiar red-winged blackbird and common grackle. A common migratory species with a range that now covers most of the continental United States and southern Canada, the cowbird is guilty of adapting too well to human-caused environmental change.

Here’s the story. Originally, cowbirds are believed to have associated closely with roaming herds of American bison. This affiliation largely confined the cowbird to the same dry, short-grass prairies that were the preferred habitat of that noble herbivore; both were species of the high plains of the interior West and Midwest.

Hanging out with the buffalo surely worked well for the cowbird. Bison, like domesticated livestock, attracted flies and other insects, which the cowbird could eat. Bison droppings were no doubt rich in undigested seeds — again, a food source for the bird. And the motion of a bison herd as it grazed across the plain stirred up still more insects, again to the cowbird’s benefit.

But the peripatetic lifestyle of a meandering buffalo herd raised one critical problem for a songbird: How to reproduce? Bird nests are by their nature fixed to one point, but the bison, effectively a moving supermarket for the bird, is not. In the course of its evolution, the cowbird was under pressure to find a reproductive method that was consistent with perpetual wandering.

The answer is a behavior called brood parasitism. A female cowbird simply dumps her eggs, one at a time, as many as 40 across a season, in the nests of other species, which she locates by following the unknowing hosts to their nests. Cowbird eggs require a slightly shorter incubation period than most birds, so often the cowbird egg hatches before the host’s own eggs. And cowbird nestlings are large and fast-growing, soon monopolizing food brought to the nest and crowding out their smaller foster siblings.

Prior to the advent of European farming culture on this continent, the parasitism of cowbirds was presumably not much of a problem. Species that shared the cowbird’s range and preferred habitats often developed some resistance to cowbird parasitism, chucking the specious egg overboard, burying it under a new floor in the nest, or otherwise destroying it with enough regularity so that the cowbird and the host species stayed in rough equilibrium.

But with the clearing or fragmentation of large swaths for forest in eastern North America, and the introduction of livestock species that were non-native but still reasonable facsimiles of bison, a vast new area of suitable cowbird habitat was created. Many of the bird species in that area were newly exposed to cowbirds, and had not had an opportunity to evolve defenses to brood parasitism. By the start of the 20th century, it was clear to ornithologists that cowbirds posed a threat to many species deemed more desirable, notably woodland birds like tanagers and thrushes.

Cowbirds have little affinity for woodland; I don’t know if the figure is still considered valid, but one study I recall found that cowbirds rarely parasitize nests more than 200 yards inside a forest. In other words, a forest fragment a quarter-mile across likely offers at least a little cowbird-free nesting area, and in large enough forests, vulnerable songbirds reproduce cheerfully with little impact from cowbirds. But with the encroachment of roads and suburbs into remaining or regenerating woodland in the East, less and less forest is remote from the open habitats favored by this avian brood parasite.

Male brown-headed cowbirds are aptly named: Glossy black for the most part, they sport a distinctly brown head. Females are among the dullest birds imaginable, uniform brownish-gray. Juveniles, fresh from the (parasitized) nest, resemble females with dark streaking. Highly vocal birds, cowbirds give a variety of gurgles and squeaks, and this gregarious species often flocks up with blackbirds or starlings.

Relative to the familiar red-winged blackbird, cowbirds are smaller, shorter-tailed, rounder-headed, and equipped with a shorter, rather finch-like bill. They rely extensively on seeds for food, especially in the colder months, and a flock of cowbirds can rapidly empty a bird feeder. Cowbirds vacate our latitude for the winter, and are among our earliest arrivals during spring migration. Numbers swell on the Vineyard in autumn, when large flocks are among the migrants that pass through.

To birders, they’re not a welcome sight, since each pair of cowbirds represents the potential failure of dozens of broods of other songbirds. (Some critical songbird populations have required lethal cowbird control measures in order to survive.) But the cowbirds, of course, are simply following their well-honed instincts, and the fact that humans now regard them as a problem stems from choices that humans themselves have made.