The brown-headed cowbird, Molothrus ater, occurs in Canada, the United States, and Mexico. It is common during summer throughout Utah, and smaller numbers are present in this state in winter, particularly in the southwestern corner of the state. It prefers somewhat open habitats, especially grasslands, such as prairies, fields, pastures, orchards, and residential areas, with low or scattered trees. Its diet consists of seeds and arthropods.
This species is a brood parasite. It does not construct a nest but lays its eggs in the nests of other species of birds, which often incubate the eggs and tend the young of the cowbird. With adequate nutrition, female brown-headed cowbirds may lay an egg almost every day during the breeding season; many females are thought to lay about 40 eggs per breeding season, and a single female has been known to lay as many as 77 eggs per season. More than 220 species of birds have been documented as being the victims of brown-headed cowbird parasitism, and 144 of these have been known to rear the young of this species. Among these many known hosts, the yellow warbler is the one most commonly parasitized by the brown-headed cowbird, and the song sparrow is second in this regard; both of these species are widespread and common breeding birds in Utah.
Human alteration of natural environments, particularly the clearing of forests and the conversion of many habitats to farm and ranch lands, has favored the brown-headed cowbird, bringing it into contact with many bird species that, under natural circumstances, it rarely if ever formerly parasitized; many of these "new" hosts for the cowbird have not evolved defensive strategies against this parasitism and are declining as a result of it.
Cowbirds get their name from their habit of associating with cattle, which they frequently follow on the ground, capturing prey such as grasshoppers that the cattle flush from the grass as they graze. Originally, this association with large grazers was with bison, mainly on the Great Plains, and the brown-headed cowbird's distribution was more limited.