The cowbird is a small (7 ½ inches long) blackbird. Males have a glossy black body and a dark brown head, while females are a dull gray-brown overall. The short, conical bill and pointed wings help to distinguish the brown-headed cowbird from larger blackbirds.
The song is a weird mix of low gurgles and high, squeaky whistles. Also gives a long sputtery trill: pt-pt-pprrrrrrrtttt! Flight call is a high, thin whistle: tsee-tseeeet!
Cowbirds are found in a variety of habitats, but they prefer woodland edges, brushy fields, and old pastures, though they are equally at home in city parks and suburban backyards. Forest fragmentation has allowed the cowbird to parasitize the nests of woodland species, such as thrushes and vireos. In winter cowbirds often join flocks of other blackbirds—red-winged blackbirds, grackles, and European starlings—foraging in fields and grasslands and roosting en masse in large woodlots.
The diet of the cowbird consists of weed and grass seeds, along with insects, especially grasshoppers and beetles. Nearly all food is taken from the ground.
Male cowbirds court females with a variety of songs, bows, and sky-pointing displays. When she is ready to lay an egg, she finds a nest that often already contains the eggs of the nest’s owner. The cowbird’s habit of laying its eggs in the nests of other, smaller songbirds makes the brown-headed cowbird a nest parasite. Cowbirds learned this behavior over centuries of following roaming herds of buffalo. The buffalo stirred up insects, the cowbird’s main food. But all the movement made it impossible to stop, build a nest, and wait for the young to grow. So the cowbirds did the most convenient thing—laid their eggs in any nest they could find along the way. This “host” nest is most frequently that of yellow warblers, song sparrows, red-eyed vireos, and chipping sparrows. The female cowbird may even remove one of the host’s eggs before depositing her own. Hatch-ling cowbirds are almost always larger than their nest mates, and are able to out-compete them for food, enhancing the cowbird’s chances of survival. Some bird species have evolved to recognize cowbird eggs and will build a new nest on top of the old one or will remove the cowbird egg.