Found on estuarine and brackish lacustrine waters. For nesting, in California preferred tree cavities near wooded mountain lakes or large streams. The California wintering population migrates to breeding grounds in Oregon, Washington, western Canada, and Alaska, and mostly is absent from April to September. Nesting territory is located on the water area nearest the nest, and is used for copulation and feeding, but the pair often feeds elsewhere as well. At lower densities, territories were larger.
Defense is mainly by the drake.
After eggs hatch, the female defends a brood territory, usually at a different site than the nesting territory, driving off other females and killing, or driving away, their ducklings. Pair formation occurs mostly on wintering grounds. Precocial young tended by the female only. Breeds first at 2 yr. Drake commonly attacks ducks of other species that enter the breeding territory.