The Ferruginous Duck, as its name suggests, is a rich, warm brown colour. The female is a slightly duller colour and a dark eye. The drake is more compact/ squat, a richer chestnut colour and with a light eye. Both have bluish grey bills with a black tip, white undersides and a white patch under their tails.
Male (above) and female (below). Diving ducks native to Central/South Europe and Asia, wintering in Mediterranean/North Africa; these Ferruginous Ducks live in London.
The bill is similar to that of the Pochard, another diving duck to which the Ferruginous Duck is related, and to the Tufted Duck. (The white patch under the tail distinguishes it from the latter.)
White-eyed Ferruginous drake
The female Ferruginous Duck is very similar to the female Rosybill Pochard; both are rich brown with white under-tail and white lower wings. The former is redder and more squat in appearance.
The latter has a slight face-shield where the top of her beak meets her face, with a flatter front line rather than the two angular prongs of the Ferruginous.