Four male Lesser Scaup in flight

I find Scaup very difficult ducks to identify because the Lesser and Greater Scaup look so much alike, and some of the field markings that are usually seen on one can sometimes be seen on the other. For example, the Lesser Scaup's head color is typically a more purple color and the Greater is typically more green, but this can vary depending on the light and the specific bird. I believe I have identified the Scaup in these photos correctly as Lesser Scaup, but if you disagree please let me know and provide any references you can.

Group of Lesser Scaup in flight. Notice the female at the front of the second group. She has the white patch at the front of her head.

The photo of the ducks in the water shows two markings. The shape of the head of the Lesser Scaup in profile has the more slowly sloping head with the tallest part at the back of the head, as we see in the photo. The Greater Scaup is  typically taller at the front of the head. The duck facing the camera shows a small black mark on the tip of the bill, indicative of a Lesser Scaup. The Greater Scaup has a somewhat larger black mark on the tip.

Four male Lesser Scaup. Not a good image, but helps ID the ducks. Notice shape of head and pattern on front of bill.

The photos of the ducks in flight show the white wing stripe stops at the bend in the wing and then turns gray. This is indicative of Lesser Scaup. With Greater Scaup the white stripe extends beyond the bend in the wing.

These photos were all taken in late February, 2012.