The Brown-Headed Cowbird

Originally a bird of the Great Plains, brown-headed cowbirds followed bison in open fields and ate the seeds and insects kicked up by their hooves. However, they have expanded their range and are now seen all over the United States. Today, cowbirds walk along cows in pastures.  In Louisiana we have three different types: bronzed, shiny, and the most common—the brown-headed.

Cowbirds forego building their own nests. Instead they parasitize the nests of other birds, usually warblers, vireos and sparrows. They toss the host egg out of the nest and lay their egg in its place. Research shows that the brown-headed cowbird can lay eggs in the nests of over 220 different species. It also suggests that individual female cowbirds have preferences for certain species over others.

While the cowbird egg is bigger and differently marked than the host egg, most of the host species do not recognize it as different from their own. The Yellow Warbler does—but is too small to knock the egg out of the nest. Instead, the warbler builds another nest over the existing one and hopes the cowbird does not return. Usually the host bird becomes a foster parent, incubating the cowbird egg and caring for that chick along with its own.

The cowbird egg will hatch more quickly than the host eggs. This imposter chick is larger than the host siblings and can raise its head higher thereby receiving the lions’ share of food from the parents. Not all of the host chicks survive. Over time this parasitism has put several species in the ‘endangered’ and ‘threatened’ category.

Early in my birding days, I hated seeing a brown-headed cowbird. Though attractive, I saw it as the villain of the bird world. It preyed on beautiful, colorful warblers, contributing to their decline. While I’m still not crazy about what the brown-headed cowbird does, I have come to see parasitism as a wonderfully creative adaptation for surviving. When you think about it, not spending energy on nest building is ingenious and frees the cowbird to lay more eggs, as many as forty in one season. Cowbirds are just doing what nature instinctively programmed them to do—to survive. The real villain is development. It has helped to expand the population and range of the cowbird.

Description: A small bird, shiny black color on the body with a rich brown colored head; the bill is conical shaped. Females are all brown or grayish, lighter colored on the head with some streaking on the belly.

When: Permanent residents in Louisiana; they form large flocks with red-wing blackbirds and European starlings in open areas: fields, meadows, pastures and lawns.

Breeding: Begins in early spring, in late March or early April. Groups of males sometimes perch together to sing and display. The male displays by spreading wings and tail, and fluffing up body feathers. Brown-headed cowbirds are not monogamous.

Feeding: Mostly feed on the ground eating grass/weed seeds and insects (grasshoppers, beetles, spiders and caterpillars). They snag the insects flushed up from grazing cows and horses in fields.

Interesting facts: Laying so many eggs, the female cowbird requires a high calcium diet. She eats snail shells and the eggshells of those eggs she has tossed from bird nests. 

Birding question? Email Harriett at harriett.pooler@gmail.com.