Courtesy Kenn Kaufman
More than 99% of Tricolored Blackbirds live in 46 of California's 58 counties, and most of the largest colonies have historically occurred in the Sacramento and San Joaquin Valleys. The range extends to the western foothills of the Sierra Nevada and Coast Ranges, and some coastal areas of northern and southern California. They also breed at scattered and isolated locations in Oregon, Washington, western Nevada, and northern Baja California. When not breeding, Tricolored Blackbirds flock along the central California coast and in the Sacramento and San Joaquin River Delta.
In the 1930s, the vast majority of Tricolored Blackbird colonies were in freshwater marshes dominated by cattails or bulrushes. By the 1970s, this tendency had dramatically changed; only about half of observed colonies were in such marshes, with others in upland or agricultural areas especially fields farmed for grain and silage. Colonies also use stands of blackberries, giant cane, tamarisk, and river-bottom trees like willow and cottonwood. Natural grasslands, dairy farms, cattle feedlots, rangeland, seasonal pools, and hay fields are used for foraging. Tricolors tend to roost in cattail marshes.
Tricolored Blackbirds forage mostly on the ground but occasionally chase insects into the air. Inserting their bills into leaves, soil, or jumbles of small rocks, sticks, or fallen vegetation, the blackbirds opens their bills to create a gap. The diet shifts as new food sources become available. Many insects are consumed: the larvae of flies, butterflies, moths, and dragonflies; agricultural pests like weevils; beetles; grasshoppers; and midges. Emergent plants and seeds are seasonally important: agricultural grains (cracked corn, oats, barley, and the sprouts of rice) and weed seeds. Hatchlings start with an insect diet, but can eat seeds within two weeks.
In late winter, male Tricolored Blackbirds begin to sing, then move into nesting areas in early spring. Flocks of males sing in a loud chorus and fly together in order to establish a colony, which sometimes displaces other blackbird species, like the Red-winged. With song and ritualized displays, individual males usually establish territories before the females arrive. A typical display features lowered and spread wings and tail, an arched neck, and ruffled neck feathers. One to four females, most often one, make a small territory within the male's larger territory. A male that attracts more than one nesting female at the same time is considered polygynous. Courtship displays and ritualized nest selection bond a pair.
Attached to standing vegetation or thin tree branches, the female weaves a cup nest of stems and leaves. For about 12 days, she incubates 3-4 pale blue or green eggs, marked with browns, purples, or black. The male evacuates the colony, only to return shortly after the chicks emerge practically naked and blind. In about a week, fledglings leave the nest to form creches. Adults encourage flight and eventually lead the young flock to form a group in an adjacent marsh. Females may produce a second brood, but often change locations to do so.
At the extremes of its breeding range, populations in northern California, Oregon, Washington, and Baja California leave the breeding grounds in winter and move into central California, but many California birds only make seasonal movements within the state.