OUT-WildlifeMoment040419

A brown-headed cowbird sits on a tree branch. The bird lays its eggs in the nests of other birds and doesn’t help raise the young. (Photo courtesy of Mike Roper)

YAKIMA, Wash. -- The brown-headed cowbird is bad news for other songbirds because it lays its eggs in the nest of other birds and takes no part in incubating its eggs or raising the young. This nesting strategy is termed “brood parasitism.” This clever ploy has had substantial negative impacts on a number of their host species, even pushing a few to endangered status.

When bison were abundant several centuries ago, cowbirds followed these roving herds across the Great Plains, feeding on insects and seeds disturbed by these mammals. It was apparently mainly a Great Plains bird until cattle spread across the continent. Nowadays cowbirds are common all across North America.

HOW TO SPOT ONE: Males of this species are appropriately named as a black bird a little bigger than a sparrow with a brown head. Females are a uniform hue of medium brown.

WHERE TO LOOK: In the nesting season, cowbirds frequent farm fields, grasslands, and woodland edges, often with open areas nearby. After nesting, they seek pastures, often with cattle.

CHOW TIME: Seeds and insects. You’ll often see cowbirds closely following cattle or horses, snapping up insects and seeds as the ground is disturbed by the big grazers. In winter, waste grain is a big source of food, especially when gleaned at feedlots.

HOME LIFE: Small groups of males and females gather near and in woodlands or field edges, where the males display by singing their odd gurgling song and posturing acrobatically. After mating the female flies stealthily through woodlands, searching for small birds that build an open cup nest, especially those of warblers and vireos. She usually lays one egg in a nest, sometimes ejecting an egg of its host before laying hers. She then departs to search for another nest, and lays another egg, repeating this for upwards of 40 times a season (exceptionally 70!). The eggs are incubated by the host species. Once hatched, the host also feeds the young, which are aggressive and often get more food from the host than the other young, and thus develop rapidly, often faster than the host species young. Because these greedy cowbird young often receive more than their share of the host birds foraging efforts, fledgling success of the host’s young is often reduced. Cowbird young leave the nest after 10 or 11 days.

Brown-headed cowbirds are documented to have laid eggs in over 220 species. The Kirtland’s Warbler of Michigan seemed headed towards extinction before biologists began an aggressive program to assist the warbler with a two-prong effort: first by trapping cowbirds and secondly, by management of the bird’s jack pine forest habitat with controlled burns. This strategy has reversed the warblers’ decline and today the population of North America’s rarest warbler has increased over ten fold to about 4,000 birds. Several other threatened and endangered species such as Bell’s and black-capped vireos and some populations of willow flycatchers have also suffered greatly from cowbirds. Habitat destruction is also a grave threat for the two vireos and efforts to reverse their population declines, even with cowbird control measures, have yet to bring about meaningful recovery of these birds.

Wildlife Moment, focusing on native wildlife, typically runs in Outdoors on the first Thursday of every month, with the cooperation of the Yakima Valley Audubon Society.