cowbird, New World bird of the blackbird and oriole (hangnest) family. The male eastern, or common, cowbird is glossy black, about 8 in. (20 cm) long, with a brown head and breast; the female is gray. Most cowbirds lay their eggs in the nests of smaller bird species, victimizing especially vireos, sparrows, and flycatchers. Sometimes the alien egg is ejected or buried under a new nest floor or the nest is abandoned, but usually the host bird incubates the egg and feeds the voracious intruder while its smaller offspring are starved or crowded out. Cowbirds eat seeds but feed chiefly on insects, following behind grazing cattle in order to capture the insects stirred up in this way—hence the name cowbird and the earlier name buffalo bird. Related birds are the bronzed, the California, the dwarf, the Nevada, and the red-eyed cowbirds. Cowbirds are classified in the phylum Chordata, subphylum Vertebrata, class Aves, order Passeriformes, family Icteridae.