Above is a photo of 2 female Greater Scaups with 1 male Greater Scaup. Unfortunately, the females are brown and drab compared to the male, this is true in a lot of cases with birds! The Male is brighter, with black and white on his body and his head sometimes has a beautiful purple hue in the right light.

Take a look at the females heads, on the Greater Scaup they are rounded at the top. Now take a look at the female Lesser Scaups below. Do you see the point at the top of the crown/head on them. That is one way (the only way for me), that you can tell the difference between the Greater and Lesser Scaups.