The first photo shows both a male Greater Scaup (on left) and a male Lesser Scaup (on right). The male Greater Scaup is larger and has a rounded head. The male Lesser Scaup is smaller and has a peaked head. The second photo shows a male and female Greater Scaup. The third photo shows a male Lesser Scaup.
GREATER SCAUP ON LEFT, LESSER SCAUP ON RIGHT
MALE GREATER SCAUP (RIGHT) & FEMALE GREATER SCAUP (LEFT)
MALE LESSER SCAUP