The brown-headed cowbird, an avian parasite
VERMONTVILLE — At first glance, the brown-headed cowbird is somewhat plain, as its name would suggest. But while the iridescent black of the male’s body may shine, the females have an even darker agenda.
Cowbirds live or breed across most of North America, and the females — unlike other birds — can lay as many as three dozen eggs a summer. So, can a single bird lay and raise that many young? Well, it doesn’t raise them.
Brown-headed cowbird females don’t bother building a nest or staking out a territory. Rather, the female will locate the nest of another bird species and lay its own eggs for the second species to hatch.
“They find nests by watching quietly for signs of other birds building nests, or they flutter through vegetation trying to flush birds from their nests. When young cowbirds hatch, they may roll the other eggs out of the nest,” the Cornell Lab of Ornithology’s website says. “Originally a bison-following bird of the Great Plains, the Brown-headed Cowbird spread eastward in the 1800s as forests were cleared. The Brown-headed Cowbird’s habit of nest parasitism can threaten species with small populations, such as the endangered Kirtland’s Warbler and Black-capped Vireo.
“Cowbirds lay eggs in a great variety of nests, including Red-winged Blackbird nests in marshes, dome-shaped Ovenbird nests on the forest floor, cup nests in shrubs and treetops, and even occasionally in nests in tree cavities. Over 140 host species of the Brown-headed Cowbird have been described, from birds as small as kinglets to as large as meadowlarks. Common hosts include the Yellow Warbler, Song and Chipping sparrows, Red-eyed Vireo, Eastern and Spotted towhees, and Red-winged Blackbird.”
There are a number of factors that lead to the success of cowbird eggs in other nests, including a shorter hatching time.
“The incubation period for cowbird eggs ranges from 10 to 12 days. The host species provides all care for the egg(s),” a report from the U.S. Forest Service says. “Cowbird eggs are laid synchronously with the host’s eggs but usually hatch first because of their relatively short incubation period. In addition, cowbirds frequently parasitize hosts whose eggs are smaller than their own; thus, the host inadvertently provides more warmth to the cowbird egg, resulting in its hatching first.”
Although the males have a distinctive brown head on a black body, the females and juveniles are much more subdued. The cowbird, which is part of the blackbird family, will often travel with related species in large flocks.
“Male Brown-headed Cowbirds have glossy black plumage and a rich brown head that often looks black in poor lighting or at distance. Female Brown-headed Cowbirds are plain brown birds, lightest on the head and underparts, with fine streaking on the belly and a dark eye,” Cornell says. “Juveniles are brown overall with a scaly-looking back and streaked underparts.
“Brown-headed Cowbirds feed on the ground in mixed-species groups of blackbirds and starlings. Males gather on lawns to strut and display for mates. Females prowl woodlands and edges in search of nests. Brown-headed Cowbirds are noisy, making a multitude of clicks, whistles and chatter-like calls in addition to a flowing, gurgling song.”
The cowbird’s behavior of damaging the reproductive success of other species has led to some efforts to remove the cowbirds themselves. However, an article on NestWatch.org says there are other factors at play.
“The cowbird does not depend exclusively on a single host species; it has been known to parasitize over 220 different species of North American birds and therefore spreads its impact across many populations,” the article says. “Although cowbirds have been implicated in the population declines of several rare species, such as Kirtland’s Warbler and Black-capped Vireo, habitat loss and fragmentation likely play a much larger role in causing songbird declines.
“This is evidenced by the fact that cowbird control alone did not increase populations of the endangered Kirtland’s Warbler; only when cowbird control was combined with habitat management for young Jack Pine forests did the warblers rebound.
“Some species, such as the Yellow Warbler, can recognize cowbird eggs and will reject them or build a new nest on top of them,” the article continues. “Those species which accept cowbird eggs either do not notice the new eggs, or as new evidence suggests, accept them as a defense against total nest destruction. Cowbirds may ‘punish’ egg-rejectors by destroying the entire nest, whereas it is possible for egg-acceptors to raise some of their own young in addition to the cowbird young.”