The foraging ecology of wintering Harlequin Ducks (Histrionicus histrionicus) is poorly understood and information on basic food habits is lacking for this species in the Aleutian Islands of Alaska where the largest winter concentrations occur. We investigated feeding behavior and food habits of wintering Harlequin Ducks in the western Aleutian Islands of Alaska with respect to sex and temporal and environmental variables to document behavioral responses to winter conditions, resource use, and nutritional requirements. We found that on average, Harlequin Ducks spent most of the diurnal period feeding (70% males, 76% females). However, more time was spent feeding during evenings, midwinter, cold weather, and high tides. Gastropods, crustaceans, and diptera larvae made up 83% of the diet, but diet composition changed throughout winter. Despite change in food habits, diet energy density was stable throughout winter.