Cowbirds are the only native American birds to always lay their eggs in nests of other species, and have the young raised by foster parents. Warblers, finches, and sparrows, all smaller than cowbirds, are the chief victims of this practice, the fast growing foster chick monopolizing food and space to the detriment of the legitimate offspring.

This is the smallest blackbird, flocking in small groups, or mixing with grackles and red-wings. They are usually quiet, their only song a faint whistle. They range north into Canada and winter in the southeastern States. Grasshoppers, beetles, and a number of insects are eaten, and like other blackbirds, they do some damage to grain.