Five things to compare for a Lesser and Greater Scaup.

1) Size, the lesser will be smaller

2) Wing coloration with the lesser white pattern stop as secondaries and the greater continues out into the primaries

3) Head color, Pruple/Green for the lesser. Green for the Greater

4) Bill and Nail. The lesse is significantly smaller

5) Back feathering pattern. The lesser will be v shaped between the wings of the shoulder color.  the Greater will be straight across