The cowbird doesn't need to build a nest or raise young-these parasitic birds, known as brood parasites, let other species do the work of raising young cowbirds.

Cowbirds, Molothrus spp. and Scaphidura spp. are parasitic birds—they lay their eggs in the nests of other birds (host birds) and let the other species feed and care for hatchlings.

How does a cowbird parasitize a nest?

Screaming Cowbirds and Giant Cowbirds usually parasitize the nests of closely related species—the Brown-headed Cowbird, very common in North America, and the Shiny Cowbird will lay eggs in the nests of hundreds of different species. Some Brown-headed Cowbirds apparently specialize in parasitizing particular host species.

Cowbirds scan for nests in which to lay their eggs. They tend to search near their foraging areas—in agricultural areas where there are abundant seeds, urban areas, and grasslands bordering forests. Thus, host species' nests in these areas are parasitized more often than nests in other habitats.

The female cowbird watches and waits until the host bird begins laying. Then she pays a quick visit to the host nest, lays an egg and leaves. Four or five eggs may be laid in the same host nest, mixed in with the host’s clutch. In some cases, cowbirds remove host eggs from the nest to make room for their own.

Recent evidence suggests that if a female Brown-headed Cowbird misses the window of opportunity and discovers a host nest after eggs have already been laid, she may vandalize the nest, forcing the host bird to start over. This would give the cowbird a second chance to parasitize that bird’s nest.

Why do other birds raise cowbird young?

Some species of birds desert nests containing cowbird eggs, or reject the hatchlings, but more than 150 different species have been observed to successfully raise young cowbirds. Cowbird eggs can look very like the eggs of host bird species. Their incubation time is short, so the cowbird hatchlings are the first to emerge and the first to start receiving food from the parent birds. Cowbird chicks, too, can look very similar to host species chicks, at least at first.

One reason host birds put up with cowbird young may be fear of reprisals: one study showed that in nests where cowbird eggs were removed, there was a higher rate of vandalism—presumably by irate cowbird parents—than in nests where the host birds incubated cowbird eggs and raised cowbird young. Although host nestlings suffered from having to compete for food with the larger cowbird nestlings, they did better overall than those in non-parasitized nests.

Are host species harmed by cowbirds?

In some areas, 90 percent of a given host species’ nests may be parasitized by cowbirds; however, this usually only occurs locally. When a bird has a limited breeding range and is heavily parasitized by cowbirds, there is a detrimental effect on the host population. For example, the Kirtland’s Warbler and Black-capped Vireo have both been heavily impacted by Brown-headed Cowbirds.

Evidence suggests that many species are not harmed by cowbird parasitism. Host young may do equally well, or even better, sharing a nest with cowbird chicks than they would in an unparasitized nest. Many species have the opportunity to nest again in the same season and bring their numbers of offspring up.

Overall, Brown-headed Cowbirds, like many other North American birds, are declining. Control programs, treating them as pest birds and using lethal measures to reduce numbers have been effective, but only locally—these measures only appear rational where host species are threatened.

Related content:

Brown-headed Cowbirds often lay eggs in meadowlark nests.

Sources:

“Cowbirds and Conservation.” Muehter, Vincent. National Audubon

Firefly Encyclopedia of Birds. Perrins, Christopher ed. Buffalo: Firefly Books, 2003

“Mafia Cowbirds: Do they muscle birds that don’t play ball?” Milius, Susan. Science News Online