The Lesser Scaup is a small North American diving duck that migrates south as far as Central America in winter. It is colloquially known as the Little Blue-bill or Broad-bill. It is apparently a very close relative of the Holarctic Greater Scaup or Bluebill, with which it forms a super species.
Adults are 45 cm long. Males weigh 850 g; females are a bit smaller and weigh noticeably less, 730 g. Wing lengths are about 20 cm
The adult males in alternate plumage have a black, effervescent head and a small tuft at the hindcrown, a black breast, a whitish-grey back and wings with darker vermiculations and black outer and geyish-brown inner primary remiges. The underparts are white with some olive vermiculations on the flanks, and the rectrices and tail coverts are black.
Adult females have a white band at the base of the bill, often a lighter ear region, and are otherwise dark brown all over, shading to white on the mid-belly.
Males in eclipse plumage look similar, but with a very dark head and breast, little or no white on the head and usually some greyish vermiculations on the wings. Immature birds resemble the adult females, but are duller and have hardly any white at the bill base. Both have white secondary remiges, a blue-grey bill with a black ''nail'' at the tip and grey feet; the males have a bright yellow iris, while that of females is orange or amber and that of immatures is brown. Downy hatchlings look much like those of related species, with dark brown upper-parts and pale buff underparts, chin, supercilium and back spots.