In the Northeast, cowbirds particularly plague warblers, vireos, flycatchers, finches, thrushes and sparrows. A female cowbird will sneak in to a nest that is temporarily unoccupied, quickly lay an egg, and fly off, sometimes after removing or eating one of the host's eggs. Cowbird eggs are whitish, with brown and gray spots. Juvenile cowbirds fledge 10 to 12 days after hatching. In one study, a successfully raised cowbird caused a reduction in the brood of a host pair by only one fledgling.
Cowbirds migrate in large flocks in spring and fall.
They winter mainly in the southern states and in Central America. Often they share huge winter roosts with starlings and other blackbirds. The adult male is chestnut and black, and the female is olive and yellow. In Pennsylvania, the species breeds most commonly across the southern part of the state. Orchard orioles feed on insects, berries, nectar and flowers.