The Brown-headed Cowbird, probably better named Bisonbird, was a bird of the open grasslands where it followed the buffalo herds. As woodlands were cleared in the eastern half of the U.S. the cowbirds were able to extend their range, which now excompasses all of the lower 48 and a good part of Canada and Mexico. Cowbirds are well known as nest parasites, the female laying up to 30 eggs a year in the nests of other birds, often of one particular species for a given female cowbird, but the list of affected songbirds exceeds 200 species. Of greatest concern are species with very limited populations, such as Kirtland's Warbler and Black-capped Vireo, which could disappear if local cowbirds proliferate.
Overall the Brown-head Cowbird population is reasonable stable now. They are protected by the migratory bird act, so local control efforts require a special permit. Some common species often hosting cowbird chicks include Red-winged Blackbirds, Song and Chipping Sparrows, and both Eastern and Spotted Towhees. Some authorities say the female BH Cowbird is under-rated as a parent, as it chooses the nest to raid carefully, and returns to check on the status of the egg and later on the chick, even in some cases bringing food. Cowbirds are promiscuous, so both males and females may have multiple partners in a given breeding season. Good source of information is Cornell Lab's "All About Birds":
http://www.allaboutbirds.org/guide/Brown-headed_Cowbird/id
Note the typical blackbird bill (yes, the cowbirds are in the blackbird family, Icteridae), which can appear silvery when the light hits at a certain angle, as well as the dark iris. The Bronzed Cowbird, invading in the southwest, lacks a brown head and has a red iris.
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