The Red-winged Blackbird is among the most abundant and commonly studied birds of North America. Since Arthur Allen's (1) ground-breaking study, an extensive ornithological literature has been produced on this species (e.g., 2, 3, 4, 5, 6, 7, 8, 9, 10, 11).
The Red-winged Blackbird is widely distributed, breeding in open wetland and upland habitats from the Pacific Coast to the Atlantic Coast, from southern Alaska and central Canada south to Costa Rica, the Florida Keys, and the Bahamas. Although primarily associated with large freshwater marshes and prairies, it also nests in small patches of marsh vegetation associated with lakes, ponds and roadside ditches, saltwater and brackish 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 resident year-round, as are populations in Middle America and the Bahamas.
Although the Red-winged Blackbird exhibits geographic variation in size, adults of all populations are sexually dimorphic in size, plumage, and behavior. The male is larger, possesses the more conspicuous definitive plumage, and is more conspicuous in behavior than 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 it one of the most highly polygynous of all bird species. Several molecular studies have shown, however, that territory owners do not necessarily sire all of the nestlings on their territories, which demonstrates that females, as well as males, copulate with more than one partner during a breeding season and even for a single nesting attempt. The species is therefore genetically polygynandrous.
The Red-winged 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 direct control measures, humans are an important source of adult mortality in this species. Population control, habitat loss, and changes in land-use and climate have resulted in a substantial decline in the continental population since 1970.