Females are mostly dark brown on the back, head, and wings with paler gray underparts. They have a small half-dome shaped patch of white feathers below the ear and a small white wing patch. The bill of the female is dark brown to black. All Buffleheads have dark brown eyes and pink legs and feet. Male Buffleheads in non-breeding plumage closely resemble the females but can be distinguished by their larger size, slightly larger cheek patch, and larger wing patch.
Juveniles are grayish-brown with small indistinct cheek patches.
Buffleheads are agile swimmers and divers but awkward on land because their legs are set well back on their bodies. In flight they are one of the fastest of all the waterfowl, and can be recognized by their small size and rapid wing-beat. They are relatively quiet ducks. Males during courtship emit a variety of sounds described as squeaks, chatters, growls, and guttural rolls. Females also employ a loud guttural call during courtship and nest site searches.