|Possible Lesser Scaup|
I am also including a reference from the Maine birding list. These problems do make birding fun. It gives your mind a workout.
"For others who may be interested, this is an instructive set of photos for testing your scaup ID skills. The photo that Rob linked (see bottom copy from his message) shows 7 scaup, but one is hidden. All the identifiable birds are Greater in that image. The next photo, however, shows all the birds strung out for easy ID. To see the details in this photo one needs to look at the “original” linked here:
Starting at the left, there’s an immature Greater, male Lesser (probably an adult), immature female Greater, adult female Greater, and then 3 adult male Greaters.
Head color is tricky and not so reliable. Under good conditions male Greater shows a dark green tinge and male Lesser a purplish sheen. I don’t think that is helpful or definitive here. The traditional head shape difference highlighted in most field guides can be tricky too. That is that Lesser shows a notch on the occiput (top rear of head/crown) and Greater is rounded. The problem is that foraging scaup compress their heads and all can look round headed. In a relaxed pose, like that shown by Rob’s birds swimming along, the classic head shape usually shows. But to get around the the notch/no notch problem, look at the crown on the Greater Scaup and see how the high point is above the eye or slightly forward of it. The head then slopes to the rear and is rounded. Also note that there isn’t much head above the eye on the Greaters. Now look at the Lesser, second from left--one can see how the forehead seems to be the lowest point of the crown and the rear the highest. That’s typical of Lesser Scaup. Also note that there is more “head” above the eye to the top of the crown on the Lesser (the eye is more central in the head but still positioned about the same point behind the top rear of the bill). Other supportive characters shown by the Lesser Scaup are its size and flank color. The male Lesser appears about 10% smaller than its flock-mates and shows duller grayish-tinged flank than the adult male Greaters. That fits for Lesser.
On the females, the immature shows a pale patch over the ear coverts. That is a good feature of Greater Scaup (esp. immature females). The adult female (bright gold eye) has her bill turned toward the camera, and one can see how broad her bill is. The wide head, wide jowls, and broad bill are really good features to look for on Greater Scaup. This is best determined by birds swimming toward you.