A train of Scaup pulled south across the water at the far end of my camera’s zoom range. The images aren’t sharp but there’s little doubt about the birds’ identity. An hour later I couldn’t see them anywhere. Possibly they took to the air again and flew further south. But two days afterward, I saw two pairs of Greater Scaup closer in on the North Basin. They were mixed in partly with the Bufflehead, which are scattered widely all over the water at this time.
The male scaup have a dark head, iridescent green in the sun, with a grey back and bright white sides that make them easily visible. The females are brown all over with what looks like a pair of white goggles over the beak; but see below.
The scaup have been seen here in much greater numbers. See, for example, the post Scaup City. Like many others, they come from the tundra lands of northern Canada. But unlike the scoters, who breed only on the North American continent, the scaup spend their summers all along the polar circle round the earth, including Europe and Asia. They’re said to be in steep decline for reasons unknown.
The Cornell bird lab writeup observes that
Occasionally an older female Greater Scaup will have male-like head color and male patterning on her back, but she still has the typical white face patch of a female.https://www.allaboutbirds.org/guide/Greater_Scaup
The photo below illustrates the observation. The bird in front is a female, as indicated by the white face patch, but her back and sides are shades of grey instead of the more usual uniform brown. The bird in back has the standard male pattern.