Lesser scaup by similar-looking "Aythya" hybrids. Except for hybrids between the two scaup species, the most reliable mark is the black bill-tip of hybrids, whereas in the scaups only the very point ("nail") of the bill is black. This is even recognizable at considerable range, as the scaups' bills appear uniformly grey from a distance, whereas those of hybrids look two-colored. European hybrids typically involve the tufted duck ("A. fuligula"), yielding offspring that have a small nape crest unlike any European "Aythya" species. Female and immature hybrids typically lack the white bill base, except in those between lesser scaup and ring-necked duck,
Lesser scaup be recognizable by the purple instead of green sheen of the head and a darker back. But this is unreliable because it varies according to light conditions, and these birds are often too far away from the observer to make out any sheen at all. The best trait—if the primary remiges are not visible—is the shape of the head: in the greater scaup drake, the forehead is usually quite massive, whereas the nape presents a smooth shallow curve and may appear almost straightly sloping. The lesser scaup drake presents the opposite shape, with a less bulging forehead and a nape