Yellow-headed Blackbirds breed across the upper Midwest, north and westward through much of lower Canada, and west and southward into Arizona and New Mexico. Wintering birds range in flocks from mid-California through the southern third of Arizona, New Mexico, and south through the farmlands of Mexico.
Yellow-headed Blackbirds breed over deep water, in freshwater marshes with cattails, bulrushes, or reeds. They avoid dense vegetation. Post-breeding flocks forage on open farmlands and prairies, feedlots, and freshwater wetlands.
During breeding, Yellow-headed Blackbirds primarily eat aquatic insects, usually found while walking along the mud at water's edge. Weed seeds and grains dominate their diet for the rest of the year. Yellow-headed Blackbirds probe with their bills, prying open and flipping over materials to reach food. Mixed-species foraging flocks appear to roll over flat land, as birds at the back fly to the front, where the ground has not yet been gleaned.
As colonial nesters, Yellow-headed Blackbird males rarely establish isolated territories, regardless of the number of females that they attract. Males and females flock to the breeding site, and males set up territories with various displays. Male Yellow-headed Blackbirds chase rival species from the colony. On average, 16 females will select a territory in a marsh with open water, plenty of channels between stands of vegetation, and vegetation that is not too dense. Although Yellow-headed Blackbird females create a "harem," they are often receptive to intruding males. Pairs do not bond.
The female weaves a cupped nest with wet vegetation that tightens as it dries. The clutch consists of 3 to 5 eggs that are greenish- or grayish-white, densely splotched with brown or darker grey. After approximately 12 days, the young hatch, and after another 12 days they fledge, at which time they move into dense vegetation, though they still cannot fly. Males help feed the young, especially in marshes that offer less food. Yellow-headed Blackbirds cooperate in attacking predators, and sometimes nest near Forster's Tern colonies for added protection and warning.
By day or night, Yellow-headed Blackbirds migrate in thin, elongated flocks. They sort themselves by gender during fall migration, with more males wintering in the northern part of their range, and more females in the south. This species joins other blackbirds at staging roosts.
Resource ~ http://birds.audubon.org/species/yelbla2