Red-crested Pochard x Scaup Netta rufina x Aythya marila
Although Red-crested Pochard is clearly one of the parents of this hybrid, the second is less certain. Scaup is a possibility but another possibility is that one parent could be this hybrid, itself not fully identified, which remains in the same collection (I have not seen Scaup there in recent visits, though birds were present until fairly recently).
