Two subspecies of Brown-headed Cowbird (Molothrus ater) are now known to breed in California. The Sagebrush Cowbird (M. a. artemisiae) has probably always been a rare summer resident in the Great Basin portions of California and a rare winter visitor in the remainder of the state (Grinnell 1915; Mailliard 1927).
Although the abundance of this subspecies has increased, its distribution has remained constant.The status of the Dwarf Cowbird (M. a. obscurus), on the other hand, has changed remarkably during the past 120 years, and it is the latter subspecies that is apparently responsible for the substantial reduction in reproductive success of a number of Central Valley and Southern California passerines.