fabalis: large,
long-necked, long-billed, flat-headed, upperparts darker than
underparts, usually extensive orange bill markings, orange legs and
feet
rossicus: small,
short-necked, short-billed, round-headed, upperparts darker
than underparts, bill with orange band, orange legs and feet
brachyrhynchus: small,
short-necked, short-billed, round-headed, upperparts (icy
blueish gray) lighter than underparts, bill with pink band, pink
legs and feet
Madge S & Burn H
(1988): Wildfowl. - London.
Ogilvie M & Young S
(1998): Photographic Handbook of the Wildfowl of the World. - London.