Both parents feed the young, which fledge after 16 to 20 days. In the fall, grackles roost in large flocks, along with starlings, red-winged blackbirds and cowbirds. Most grackles winter to the south of us, but some stay on here. The brown-headed cowbird is a bird of farms, fields and woods edges. Seeds of grasses and weeds, plus waste grains, make up about half of the birds' diet in summer and more than 90 percent in winter.
Cowbirds also eat insects, particularly grasshoppers, beetles and caterpillars.
The species builds no nest. 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.