Editing Lesser scaup (section)

===Identification===
<gallery>
File:Lesser Scaup 2.jpg|Lesser scaup drake in basic plumage. Note typical head shape; purple sheen visible on neck.
File:Greater scaup male.jpg|Greater scaup drake in basic plumage. Note typical head shape; green sheen visible on neck.
File:Aythya affinis.JPG|Lesser scaup hen.
File:Greater scaup female.jpg|Greater scaup hen.
</gallery>

Lesser scaup are often hard to distinguish from the [[greater scaup]] when direct comparison is not possible, but in North America a large [[Aythya|scaup]] flock will often have both species present. Females, juveniles and drakes in eclipse plumage are hard to identify; there is considerable overlap in length between the two species, but greater scaup are usually noticeably more bulky. Lesser scaup females and immatures tend to have less white around the bill, but this too varies considerably between individual birds. Lesser Scaup generally have a smaller and straighter bill, with a relatively narrow profile, while Greater Scaup have a wider, more spatulate profile toward the tip of the bill. The dark nail tip averages wider on Greater Scaup.<ref>{{Cite book|last=Sibley|first=David|url=https://www.worldcat.org/oclc/869807502|title=The Sibley guide to birds|date=2014|others=Scott & Nix|isbn=978-0-307-95790-0|edition=2|location=New York|oclc=869807502}}</ref> In flight, the most tell-tale sign is the white secondary [[remiges]], whereas in the greater scaup the white extends on the primary remiges also, i.e. far towards the wingtip.<ref name=DUb/><ref name = carboneras1992 /><ref name = Madge/><ref name=DUa/> 

Lesser scaup drakes in nuptial plumage are often said to 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 shape of the head is a crucial difference, but only when birds are at rest and not actively diving. In the greater scaup drake, the forehead is usually quite massive (especially in North American and East Asian populations), 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 that looks strongly curved or even angular due to the small crest. When the birds raise their heads, these differences are most easy to spot, and after observing the two species in direct comparison it usually becomes easy to recognize. In fact, in basic plumage the lesser scaup drake may appear identical in shape and size to a drake of the [[ring-necked duck]] (''A. collaris''); the black back and wings of that species are hard to confuse with the light ones of the lesser scaup male though.<ref name="DUb" /><ref name="carboneras1992" /><ref name="Madge" /><ref name="DUa" />

====Hybridization====
Particularly in the case of vagrant birds in Europe, the identification is complicated by similar-looking ''[[Aythya]]'' [[hybrid (biology)|hybrid]]s. 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, where the white extends to the eye region. But especially with juveniles, the bi-colored bill of hybrids is most diagnostic.
Hybrid combinations that are known from the wild and resemble the lesser scaup are:
* The occurrence of hybridization between lesser and greater scaup in the wild is disputed. Such hybrids could only be identified with certainty by [[DNA sequence]] comparison however. But while they may exist unnoticed, they cannot be frequent, as the species are largely [[sympatric]] and closely related, yet remain distinct, with no signs of significant [[introgression]].
* Hybrids between lesser scaup and ring-necked ducks are recognizable by very dark wings contrasting with a light grey underside more than in the lesser scaup but less than in the ring-necked duck.
* Hybrids between the lesser scaup and the [[redhead (duck)|redhead]] (''A. americana'') are recognizable by the lack of contrast between wings and belly and the dull brownish head.
* Hybrids between the tufted duck and the [[common pochard]] (''A. ferina'') are almost indistinguishable from lesser scaup, though neither parent species resembles ''A. affinis''.
In theory, each and every ''Aythya'' species is able to produce potentially [[fertility|fertile]] hybrids with any other, though due to their different ranges and behavioral cues given during courtship most of these hybrids are only known from birds kept in captivity without [[conspecific]] mates.<ref name = carboneras1992 /><ref name=Madge/>