Ornithologists believe that cowbirds did not live in forested Pennsylvania before European settlement, a theory bolstered by the fact that few of our native songbirds have evolved defense behaviors against its parasitism. Today, cowbirds are common breeders statewide, mainly in farmland and in areas where development has fragmented the forest, giving them access to the nests of woodland birds. Cowbirds have been reported to parasitize more than 220 different species. 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.