Wild Ideas: Villain or hero, the brown-headed cowbird is unique

Common hosts of cowbird eggs

One way to find where cowbirds are most likely to lay their eggs is to find the nests of those species. The cowbird eggs are white to grayish-white with brown or gray spots and are generally larger than their hosts’. The following are the top host species; most are common to Rappahannock County:

Eastern phoebe
Wood thrush
Red-eyed vireo
Warbling vireo
Yellow warbler
Common yellowthroat
Yellow-breasted chat
American redstart
Red-winged blackbird
Northern cardinal
Indigo bunting
Eastern towhee
Spotted towhee
Chipping sparrow
Song sparrow

We humans have a propensity for labeling animals as heroes and villains — but usually this reflects  more on us than on the animals. Such is the case with the cowbird. Often seen as a villain, to some extent it has become one, thanks to the dramatic changes we’ve made to the land.

The cowbird’s villainous reputation is based on its habit of laying its eggs in the nests of other birds. The host birds then foster the cowbirds’ chicks, raising them as their own. Cowbirds are, in fact, the only North American bird species that cannot build nests of their own and instead parasitize the nests of other birds. Of the three North American species of cowbird, which are in the same bird family as blackbirds, the brown-headed cowbird (Molothrus ater) is the only one native to the eastern United States.

Cowbirds were once known as “buffalo birds” because they followed herds of bison and fed on the insects churned up by their movements, according to the Smithsonian National Zoological Park website. The cowbird’s nomadic life perhaps accounts for its amazing reproductive adaptation. Establishing a territory, building a nest, laying and incubating eggs and raising young are all hard to do when you’re constantly on the move. The answer for the cowbird was to get foster parents to do the job.

Cowbirds have been found to parasitize the nests of more than 220 species, with 144 documented to have reared cowbird young, according to the Cornell Lab of Ornithology’s newsletter, “BirdScope” (November 2008). “Recent genetic analyses have shown that most individual females specialize on one particular host species,” adds AllAboutBirds.org. According to the Stokes Nature Guide “A Guide to Bird Behavior,” the most common of these hosts are warblers, vireos, flycatchers and finches.

The female cowbird looks for nests in which to lay her eggs primarily in the morning, says the Stokes guide. She uses three main strategies: Perching in trees and quietly watching the activities of host birds, creeping along the ground or in low shrubbery to spot host birds building nests, or flying repeatedly at shrubbery to get potential hosts to reveal the location of their nests. The best way to find where a cowbird will lay an egg is to follow her or watch who she’s watching.

A male American goldfinch, in his bright-yellow breeding plumage, shares a bird feeder with a drabber female brown-headed cowbird, apparently unaware or unconcerned that cowbirds like to lay their eggs in finch nests.
A male American goldfinch, in his bright-yellow breeding plumage, shares a bird feeder with a drabber female brown-headed cowbird, apparently unaware or unconcerned that cowbirds like to lay their eggs in finch nests. Pam Owen | Rappahannock News

Cowbirds tend to choose nests containing eggs smaller than their own, says AllAboutBirds.org. The Smithsonian website adds that, in approximately 60-70 percent of nests parasitized, the cowbird removes one of the host eggs before laying her own. The website suggests that this might be to ensure that the host does not notice the number of eggs has changed and that the host accepts the cowbird’s egg and fully incubates it.

Not only is the cowbird egg larger than the host bird’s, its incubation period is also shorter, and the early growth rate of hatchlings is more rapid than most of the species cowbirds parasitize. Early to mature, cowbird chicks usually also leave the nest before the host’s young do.

So why don’t host birds just throw out or destroy the cowbird’s egg? Most hosts “don’t recognize cowbird eggs at all,” says AllAboutBirds.org. And some species can recognize the invader’s egg but have varied success dealing with it. The yellow warbler, for example, can recognize the egg but is too small to throw it out. Some host birds “build a new nest over the top of the old one and hope cowbirds don’t come back,” says AllAboutBirds.org. Only nine host species have been determined to be rejecters, either tossing the cowbird’s egg out or puncturing it; most of these are “large birds with beaks large enough to remove unwanted eggs easily,” according to the Smithsonian website.

In what seems like an unnecessary act of villainy, “when a host bird does manage to remove a cowbird’s egg, the cowbird may return to destroy the remaining eggs,” says the BirdScope article. Perhaps the cowbird has evolved to do this in an attempt to stop that genetic line and provide space for a more favorable host to take its place.

Male brown-headed cowbirds are more striking than their mates, with glossy black bodies and brown heads.
Male brown-headed cowbirds are more striking than their mates, with glossy black bodies and brown heads. Pam Owen | Rappahannock News

While parasitizing others’ nests may not seem fair to us humans, cowbirds pay a price for not sticking around to raise their young. According to the Stokes guide, a female cowbird can lay a whopping 40 eggs each year on average, but only two or three mature to adulthood.

In contrast, our eastern phoebes generally raise two broods of three to four young each a year. In my experience, with the exception of nests that become infested with parasites, most or all of the chicks in these broods survive to at least the fledgling stage. And one study cited in the Stokes guide showed that a successfully raised cowbird reduces the brood of the host by only one fledgling.

This past week I was housesitting for friends in Fauquier County and regularly saw two females and three male cowbirds at various times at a bird feeder. What surprised me was that goldfinches and chipping sparrows, among the preferred hosts of cowbirds, were feeding right next to their foe, apparently oblivious to the threat the latter poses.

Once limited to the short-grass plains of the Midwest, cowbirds have been able to expand their range through changes we humans made to the land. As the BirdScope article puts it:

“Many ornithologists believe that cowbird numbers were once limited by high winter mortality and scattered bison herds. But when people decimated bison, they introduced cattle and started growing crops such as winter wheat, vastly increasing cowbird winter survival and giving their burgeoning numbers an ever-expanding range.”

At the same time, in the East, forests were being cleared for farms, then suburbs and cities, providing more open space for the cowbird. Cowbirds can easily fill these spaces because they are generalists when it comes to food and habitat needs.

When cowbirds have parasitized the nests of bird species that are at risk, control has been attempted through trapping and shooting but, according to the Smithsonian website, the practice is controversial. Unless we convert much of the open areas we’ve created back to forest, cowbirds are likely here to stay.

© 2014 Pam Owen

Pam Owen
About Pam Owen 343 Articles
Writer, editor, photographer, and passionate nature conservationist living in Rappahannock County, in the Blue Ridge Mountains of Virginia. Two favorite quotes: By E.O. Wilson, who coined the term "biodiversity," "Nature holds the key to our aesthetic, intellectual, cognitive and even spiritual satisfaction”; by Douglas Adams, “I love deadlines. I love the whooshing sound they make as they pass by.”