Cowbirds need to feed in open fields. Long ago you would have found them in the grassy plains they shared with the buffalo. When settlers arrived and cleared forests for agriculture, the cowbird population increased several times over. Cowbirds flocked to the newly opened spaces and thrived. Today, over a hundred years later, the cowbird range covers almost all of North America south of the Arctic Circle and is five times larger than it was in the last century.
Cowbirds quickly move into any area where people cut trees or plant crops.
Cowbirds do not nest or raise their young. Female cowbirds simply lay their eggs in other birds' nests hoping that the unsuspecting host will care for the cowbird egg as if it was its own. This is called nest parasitism and often works at the expense of the host's nestlings. Where there are large tracts of unbroken wilderness, cowbirds are not a problem. When open areas favored by them are created beside forests, the nesting habitat quality of the forest is reduced by cowbird nest parasitism.