Lesser Scaup
Aythya affinis

f22 @ 1/500s, ISO:1600, Nikon D3S w 500mm

"Lesser Scaup," Wikipedia, the free encyclopedia. Lesser scaup are often hard to distinguish from the greater scaup when direct comparison is not possible, but in North America a large scaup flock will often have both species present. Females, juveniles and drakes in eclipse plumage are hard to identify; there is considerable overlap in length between the two species, but greater scaup are usually noticeably more bulky. Lesser scaup females and immatures tend to have less white around the bill, but this too varies considerably between individual birds. The bill may give a hint; in the lesser scaup it has a stronger curve on the upperside than in the greater, resulting in a distal part that looks somewhat flattened and wide in the lesser scaup—hence the vernacular name "broadbill". If the birds fly, the most tell-tale sign is the white secondary remiges, whereas in the greater scaup the white extends on the primary remiges also, i.e. far towards the wingtip.
Salt Pond, Falmouth, Massachusetts