Wildlife Watching Wednesday: Cowbird Brood Parasitism

by Tom Berg

Brown-headed cowbirds are unusual birds. They are just one of many species of native blackbirds, and as the name says, even though the body feathers of the male birds are black, they have a noticeably brown head. Females of the species are much more plain; they are just an ordinary grayish-brown color overall.

These cowbirds are unusual because unlike most birds that build a nest and then lay eggs in it, they lay their eggs in the nests of other birds and let the host birds raise their young. This is called brood parasitism, and brown-headed cowbirds are well-known as brood parasites. Female brown-headed cowbirds never make their own nests and always lay their eggs one at a time in the nests of other birds.

Although some bird species fight back by pushing the cowbird egg out of their nests or puncturing the alien egg, other species (such as the northern cardinal) never seem to notice. Those oblivious birds incubate the cowbird egg along with their own eggs, and then raise the young cowbird as their own. Over 200 species of birds are known to have their nests parasitized by brown-headed cowbirds.

One of the many problems with this brood parasitism is that the cowbird eggs usually hatch faster than the eggs of the host species, so the young cowbird gets a head start on the other hatchlings and often monopolizes the food from the “parent” birds. This allows them to grow faster, and they sometimes bully their non-cowbird siblings and even push them right out of the nest.

Brown-headed cowbirds are not always successful, however, especially if they lay an egg in the nest of a goldfinch. Goldfinches are strictly vegetarian and they only eat seeds. So when a cowbird lays an egg in a goldfinch nest, the cowbird hatchling quickly dies because it cannot survive on the seed-only diet that goldfinches provide for their young.

I guess that proves the brown-headed cowbird doesn’t always win!

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