The Tricolored Blackbird is California's blackbird -- more than 99% of the population occurs within the state. The geographic range of this species is restricted to California's Central Valley and surrounding foothills, a few remaining coastal and inland localities in southern and central California, scattered sites in Oregon and central Washington, a single site in western Nevada, and western coastal Baja California (Figure 1).
This blackbird is 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 exhibit a wide range of foraging behaviors, nest in a variety of substrates, and consume a diverse array of seeds, ripening grains, and arthropods. They are sympatric with and morphologically similar to Red-winged Blackbirds (Agelaius phoeniceus), but unlike the Red-wingeds, Tricoloreds breed in dense colonies and may travel several kilometers to secure food for their nestlings. Also unlike Red-winged Blackbirds, male Tricoloreds defend small territories used only for breeding and mate with 1 to 4 females.
The Tricolored Blackbird forms the largest colonies of any North American passerine bird. Breeding colonies have historically attracted tens of 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, suitable nesting substrate, and open-range foraging habitat of natural grassland, shrubland, or agricultural cropland. In winter, Tricoloreds may form single-species, and sometimes single-sex, flocks, but they commonly forage with other blackbird species and roost in multi-species flocks. They often change their nesting locations from year to year, likely an adaption to exploit rapidly changing environments in ephemeral habitats, enhancing prospects for finding secure nesting sites and relatively plentiful insect food supplies. They are itinerant breeders, nesting more than once at the same or, more commonly, in 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 that often affect thousands of nests at a single colony. 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 and an additional decline of 35% between 2008 and 2011 were identified by intensive statewide population surveys. Historically, this species was harvested as food for miners and residents of urban areas and, until the 1980's, it was killed by farmers attempting 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 Wildlife, Tricolored Blackbirds experience large annual breeding losses to crop-harvesting activities and insufficient insect food, and suffer habitat losses to land conversions from rangeland to vineyards, orchards, other agricultural crops, and urban development.
Key studies include those covering socioecology (Orians 1961b, Orians 1961a, Collier 1968), vocalizations and behavior (Collier 1968, Orians and Christman 1968), reproductive biology and physiology (Payne 1969), population trends (Dehaven et al. 1975a, Hamilton III et al. 1995), distribution and migration (Neff 1937, Neff 1942, Dehaven et al. 1975b), and effects of insect abundance on reproductive success (Meese 2013). Recent population censuses (Kelsey 2008, Kyle and Kelsey 2011) indicate that the Tricolored Blackbird is increasingly conservation-dependent, and continuing population declines warrant active management of its breeding and foraging habitats and more intensive study of its winter ecology.