Introduction

Tricolored Blackbirds are largely native to California (which is home to more than 99% of the population). The geographic range of the Tricolored Blackbird is restricted to the Central Valley and surrounding foothills, throughout coastal and some inland localities in southern California, and scattered sites in Oregon, western Nevada, central Washington, and western coastal Baja California (Fig. 1). They are sexually dimorphic in size, plumage, and behavior. Males are larger than females, possess striking red, white, and black plumage, and display conspicuously when breeding. They are sympatric with Red-winged Blackbirds (Agelaius phoeniceus). Unlike Red-winged Blackbirds, however, Tricolored Blackbirds breed in dense colonies and may travel several kilometers to secure food for their nestlings; males defend small territories within colonies and mate with 1 to 4 females.

The Tricolored Blackbird forms the largest colonies of any North American passerine bird. Breeding colonies may attract thousands of birds to a single site. In the 1930s, 1 colony was estimated to include more than 200,000 nests. Colonies require nearby water, a suitable nesting substrate, and open-range foraging habitat of natural grassland, woodland, or agricultural cropland. In winter, they often form single-species, and sometimes single-sex, flocks, but they also flock with other blackbird species. They often change their nesting locations from year to year. These changes may be an adaptation to exploit rapidly changing environments in ephemeral habitats, providing secure nesting sites and plentiful insect food supplies. They are itinerant breeders, nesting more than once at different locations during the breeding season.

The status of the Tricolored Blackbird is of concern because its population has declined and its colonial nesting behavior makes it vulnerable to nesting failures affecting thousands of nests at large colonies. Studies in the 1970s reported that the overall population was greatly reduced from that observed during the 1930s. A decline of 37% between 1994 and 1997 was identified by intensive population surveys. Historically, this species was killed to control damage to rice and grain crops. Currently considered a Species of Concern by the U.S. Fish and Wildlife Service and a Species of Special Concern by the California Department of Fish and Game, Tricolored Blackbirds experience large annual losses of reproductive effort to crop-harvesting activities and suffer habitat losses to land conversions from rangeland to vineyards, orchards, and urban development.

Key studies include those covering socioecology (Orians 1961a, 1961b, Collier 1968), vocalizations and behavior (Collier 1968, Orians and Christman 1968), reproductive biology and physiology (Payne 1969), population trends (DeHaven et al. 1975a, Hamilton et al. 1995), and distribu-tion and migration (Neff 1937, 1942, DeHaven et al. 1975b). Recent population censuses (Beedy and Hamilton 1997) and itinerant breeding considerations (Hamilton 1998) indicate that continuing population declines warrant active management of this species and its breeding, foraging habitats, and possibly its unstudied winter range.