Adults usually reach a length of 30 to 37 cm. Yearling males may be mistaken for females. They winter in the Bay and along the Atlantic coast as far north as New England and begin to arrive in the Bay in the fall. Their numbers peak in November, as many migrants arrive from their summer breeding grounds in Canada and the northern tier of the United States, where they nest by woodland ponds and streams. Buffleheads often nest in trees, in abandoned flicker nest cavities.
Females are small enough to pass through the flicker hole.
The nests usually consist of loose material and vegetation and are lined with down from the hen's breast as she lays and incubates her eggs. Buffleheads occasionally will nest in sandbanks, like kingfishers. Females travel north every year between mid-April and May to nest in the area in which they were born. During breeding season, they lay one egg each morning for one to two weeks. The male bufflehead will abandon the female when full-time incubation begins, and spend the summer at bufflehead molting grounds.