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Three subspecies are currently recognised, split into two species by several authorities:
M. f. fusca (Velvet Scoter) is found in Europe and western Asia.
M. f. deglandi (White-winged Scoter) occurs in North America and includes M. f. dixoni which is no longer recognised.
M. f. stejnegeri (Siberian Scoter) is found in Eastern Asia. This form is also sometimes proposed for recognition as a full species.
M. f. deglandi has in the past and is increasingly again recognised as a full species (for example by the British BOURC), which would keep the name of White-winged Scoter; subspecies M. f. fusca being Velvet Scoter. The subspecies M. f. stejnegeri is a subspecies of M. deglandi if IOC or BOURC is followed, but is also considered a full species, M. stejnegeri by BirdLife.
All three subspecies are prone to occasional vagrancy, with a scattering of records of birds in the normal ranges of the other subspecies.
Notes on distinguishing males of the subspecies
Velvet Scoter with wing part-open showing the white secondaries Photo by Digiscoper321 Western Sweden, April 2015
M. f. fusca: the least knob on the bill, and the least white around and behind the eye; almost no white above the eye. The coloured section of the bill is yellow and relatively long, and the distance from the base of that to the eye is short, producing the impression that the eye is positioned relatively far forward on the head. The crown looks highest above the eye.
M. f. deglandi: clear knob at the base of the bill, with the yellow-orange part relatively short, so that the eye looks set further back on head than M. f. fusca. The white around the eye is shaped as a checkmark, pointed up at the rear end. The coloured part of the bill is orange with the top parts looking yellow. The crown is highest in front of the eye. When seen well, flanks will be brown in contrast to black back and breast.
M. f. stejnegeri: Head shape closer to Common Eider in shape, with a long, sloping forehead. White around eye similar to or longer than M. f. deglandi. It has a clear knob - almost like a small Rhinoceros horn - on the bill, further forward than M. f. deglandi, and the coloured parts are mostly red with yellow "lipstick" below. Flanks are black.
Females are much harder to distinguish, only determinable at the closest range; M. f. fusca has a slightly concave forehead with no basal swelling on the bill, M. f. stejnegeri a slightly swollen bill base, and M. f. deglandi a marginally more swollen bill base.