It's very hard to say! My initial thought was that this is a female Lesser Scaup (Aythya affinis) based on the shape of the back of the head and the white at the base of the bill. However, it has a proportionally large bill like that of a Greater Scaup (Aythya marila) and the head is not particularly dome-shaped. My gut says Lesser Scaup, which also is the more common of the two species in U.S. collections, but I'm not 100% certain based on only one photograph.
