Steller’s eiders are the smallest of the four eider species, averaging 43-47 cm (17-18.5 in.) with average body mass of about 850g (female) or 880g (male) (about 2 lbs.). Adults of both sexes have a blue patch with a white border on the upper wing, similar to a mallard.
In winter, spring, and early summer adult males are in breeding plumage with a black back, white shoulders, chestnut breast and belly, a white head with a greenish tuft, and small black eye patches. During the late summer and fall, males have mottled dark brown body plumage but retain the white wing bar. Females and juveniles are mottled dark brown year-round.