We think of a parasite as an organism that lives on or in another organism and derives its food from its host. That’s the common definition, but in use it’s generally broader. Your brother-in-law who constantly borrows money and doesn’t pay it back could be considered a parasite.Among birds, we have what are called brood parasites – those birds that put their eggs in another bird’s nest and let that bird raise the parasite’s young. There are about 100 species of birds that are considered brood parasites, including indigobirds, whydahs, cowbirds. Long-tailed Shrike, some laughingthrushes, European Roller, some magpies, ravens, and starlings, laughingdove, some bulbuls, and the Black-headed Duck, at least 100 species.
In North America, all we have in this category is the Brown-headed Cowbird, which lays its eggs in the nests of about 225 species of birds. Cowbirds don’t specialize on any one particular species, but an individual female usually does. The female cowbird watches a nesting host and when the coast is clear, flies to the nest, removes a host’s egg and replaces it with one of her own, all in a matter of seconds. The host bird unknowingly raises the young cowbird, which usually outcompetes the other young in the nest because the cowbird has a shorter incubation time and grows more rapidly than the host young. A female cowbird might lay as many as 40 eggs in a season. Having a great need for calcium to produce their own eggs, female cowbirds will eat snail shells as well as the shells of eggs in the nest they parasitize.
Although the cowbirds make have a detrimental effect on some species, especially those that are threatened or endangered, cowbirds are certainly less harmful than habitat destruction and climate change.
Not all host birds make good parents. Some recognize the cowbird intruder and eject the egg, destroy it, cover it with another nest, or abandon the nest altogether. Others simply ignore the cowbird chicks and let them perish.
Since cowbirds do not build their own nests, their reproductive behavior is different than the typical songbird. Males will display to females with spread wings and a song and pose aggressively towards other males. Brown-headed Cowbirds are not monogamous and both males and females might have several mates in one season. When the young leave the nest (fledge), they are still fed by their foster parent. The young cowbirds often form groups of juvenile birds where they apparently learn to socialize the cowbird way.
Being raised by another bird, they are not exposed to the song of their parents. But after they leave the nest and begin their adult life, the young cowbirds are especially sensitive to the song of their species and learn it that way. Watch and listen here.
The males are black with a brown head and the females a dull brown. They are called cowbirds because at one time they used to follow herds of buffalo, eating the insects that the bison stirred up along with seeds. Today they follow horses and cattle. The scientific name Molothrus ater means “full black greedy fellow.”