The Tricolored Blackbird is California's blackbird—more than 99% of the global population occurs within the state. The geographic range of this species is restricted to California's Central Valley and surrounding foothills, coastal and inland localities in southern and central California, scattered sites in northeastern California, Oregon, and central Washington, a single site in western Nevada, and locally in northwestern Baja California (Figure 1).
This blackbird species 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. The species nests in a variety of substrates, exhibits a range of foraging behaviors, and consumes a diverse array of arthropods, seeds, and ripening grains. It is sympatric with and morphologically similar to the Red-winged Blackbird (Agelaius phoeniceus), but unlike the Red-winged, the Tricolored Blackbird breeds in dense colonies and may travel several kilometers to secure food for nestlings. In addition, the male Tricolored Blackbird defends 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 species. Breeding colonies historically attracted tens or even hundreds of thousands of birds. In the 1930s, a single colony was estimated to include more than 200,000 nests (about 300,000 adults) and cover almost 24 ha (1). Colonies require suitable nesting substrate surrounded by foraging habitats that may include semi-natural grasslands, agricultural croplands, or alkali scrub habitats, and a nearby source of freshwater. In winter months, the Tricolored Blackbird may form single-species, and sometimes single-sex, flocks, but the species commonly forages and roosts together with other blackbird species. Nesting locations often change from year-to-year, perhaps an adaption to exploit ephemeral habitats or high abundance of arthropod resources, or to enhance prospects for finding secure nesting sites. The species is an itinerant breeder, nesting more than once at the same or, more commonly, different locations during the breeding season.
The conservation of the Tricolored Blackbird is a matter of increasing concern owing to population declines, and because the species' habit of nesting in large colonies make it more vulnerable to nest failures that can 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. More recently, intensive population surveys in California identified a decline of 37% between 1994 and 1997, and a 63% decline between 2008 and 2014, followed by an increase of 22% in 2017. Historically, this species was harvested as food for miners and residents of urban areas, and shooting by farmers attempting to reduce damage to rice and other grain crops in the Sacramento Valley has been documented since 2007 (RJM, personal observation). The Tricolored Blackbird has experienced high annual breeding losses due to crop-harvesting activities (except in 2016) and insufficient insect resources, and habitat loss resulting from conversion of rangeland to vineyards, nut orchards, other agricultural crops, and urban development.
In April 2018, the California Fish and Game Commission made a final determination to list the Tricolored Blackbird as Threatened under the California Endangered Species Act. It is currently considered a Species of Concern by the U.S. Fish and Wildlife Service, and it is under formal status review for listing as Endangered under the Federal Endangered Species Act (2).
Key studies have examined aspects of socioecology (3, 4, 5), vocalizations and behavior (5, 6), reproductive biology and physiology (7), population trends (8, 9, 10, 11), distribution and migration (1, 12, 13, 14), and reproductive success (15, 16). Population declines as indicated by recent censuses (17, 18, 11) suggest that the Tricolored Blackbird will be increasingly dependent on conservation actions, including active management of breeding and foraging habitats, and more intensive study of overwintering ecology.