Greater and Lesser scaup are closely related ducks that are hard to tell apart. Greater Scaup, like in the first photo, have large, black, bill nails and rounded heads. The head may be slightly highest in the front, but head shape often depends on what the duck is doing (diving, resting, etc.). The head often shows a green sheen, but this field mark depends so much on light conditions that color is not a dependable trait. The back is finely barred, making it appear brighter white than in the Lesser Scaup. Finally, the sides of the Greater Scaup often appear to be brighter white.
Compare the Greater Scaup to the duck in the second photo. The bill nail is much less prominant, barely visible from the side. The head is slightly crested behind the head. The black back barring is more pronounced, and the sides are not as bright white. The head sheen appears to be purple, not green.
So what is this last scaup? The bill nail is not pronounced and the head is clearly purplish. On the other hand, the head is not crested and appears to be highest in front. The back and side seem to be very pale. I have an opinion about this bird’s identity, but I welcome reader input. Perhaps the correct guess may be “unknown species of scaup.”
I took the first and the last photos at the Minnesota Valley National Wildlife Refuge’s Bass Ponds last Friday, 11 March 2016. I do not recall where I took the Lesser Scaup photo. All these ducks are drakes. Identification of the hens requires even more finesse.