The Red-winged Blackbird is perhaps the most abundant and most commonly studied bird of North America. Since Arthur Allen's ( Allen 1914b ) ground-breaking study, an extensive ornithological literature has been produced on this species (see Nero 1984 , Orians Orians 1980 , Orians 1985 , Searcy and Yasukawa Searcy and Yasukawa 1983 , Searcy and Yasukawa 1995 , Orians and Beletsky 1989 ).
The Red-winged Blackbird breeds in marsh and upland habitats from southern Alaska and central Canada to Costa Rica, and from California to the Atlantic Coast and West Indies. Although primarily associated with large freshwater marshes and prairies, it also nests in small patches of marsh vegetation in roadside ditches, saltwater marshes, rice paddies, hay fields, pasture land, fallow fields, suburban habitats, and even urban parks. This blackbird migrates to and from the northern portions of its breeding range, but some populations in the western United States and Gulf Coast are known to be resident year-round, as are populations in Middle America.
Although the Red-winged Blackbird varies in size geographically, adults of all populations are sexually dimorphic in size, plumage, and behavior. The male is larger, possesses the more conspicuous definitive adult plumage, and is more conspicuous in his behavior than is the female. In addition to its striking sexual dimorphism, the Red-winged Blackbird is also known for its polygynous social system. Up to 15 females have been observed nesting on the territory of a single male, making this one of the most highly polygynous of all bird species. Recent molecular studies have shown that territory owners do not necessarily sire all of the nestlings on their territories, which demonstrates that females as well as males often copulate with more than one partner during a breeding season and even for a single nesting attempt.
This blackbird is also known for its membership in huge, mixed-species roosts that form during the nonbreeding season and for its ability to damage important crops such as corn, sunflower, and rice. Considerable effort, time, and money have been spent attempting to control blackbird roosts and to reduce crop damage. As a result of such control measures, humans are now one of the major sources of adult mortality in this species.