The bufflehead rivals the green-winged teal as the smallest American duck.
Buffleheads have evolved their small size to fit the nesting cavity of their "metabiotic" host, a woodpecker, the northern flicker. Due to their small size, they are highly active, undertaking dives almost continuously while sustained by their high metabolism. They do not tend to collect in large flocks; groups are usually limited to small numbers. One duck serves as a sentry, watching for predators as the others in the group dive in search of food. Buffleheads are amongst the last waterfowl to leave their breeding grounds and one of the world's most punctual migrants, arriving on their wintering grounds within a narrow margin of time.
They nest in cavities in trees, primarily aspens or poplars, using mostly old flicker nests, close to water. Nest competitors include mountain bluebird, tree swallow, and European starling