The male yellow-headed blackbird is extremely territorial during the breeding season (2), which runs between May and June (4). The male arrives on the breeding grounds before the female (2) (4) to establish its territory, which is usually between 100 and 600 square metres (4). Territories are not created during winter, when large flocks are formed (2).
The polygamous male yellow-headed blackbird can have up to 16 females in its harem. Some males may not have any females enter their territory during the breeding season, and will go into the territory of another male to mate (2). The female selects the nesting site and builds the nest (2) (3), which is a cup-shaped structure built directly over water from long strips of plant material that are woven between the leaves of aquatic vegetation (2) (4). The nest usually takes between four and five days to complete (4). The average clutch of this species contains two to five eggs (3) (4), which are green-white or grey-white with brown markings across the surface (2) (3). The eggs are incubated by the female for 12 or 13 days (2) (3) (4), and once they have hatched, the female is mostly responsible for feeding the young. Generally, the more females that a male has in his harem, the less it feeds the young (2) (4) and the male may only feed those in the first nest that was built in its territory (3). The young remain in the nest for up to 14 days (3) (4), but continue being fed by the female after leaving the nest (2).
In late August or September, the yellow-headed blackbird leaves its breeding grounds (2) (4) in single-sex flocks to begin its southward migration. The female migrates further south than the male, although both sexes generally arrive in their overwintering grounds between August and September. Northward migration to the breeding grounds usually occurs in February or March (4).
During the breeding season, the diet of the yellow-headed blackbird consists mainly of aquatic insects (2) (3), and in winter it mostly takes seeds and grains such as wheat, oats and maize (4) from agricultural areas (2) (3) (4).