Look at the structural differences between the bill of the hen Lesser Scaup in the foreground and the bill of the male Greater Scaup in the background. The small narrow bill of the Lesser is really over powered by the huge bill on the male Greater. The Lesser also shows the peak on the crown behind the eye and the Greater shows a large round head shape but unfortunately I cut the rounded crown off this shot. One word of caution, the peak on the Lesser Scaup's head can disappear on a diving bird, giving it a round headed look.