In all plumages, Ferruginous Duck is essentially dark brown with white undertail coverts.
Adult males are distinctively white eyed, whereas females and juveniles have brown eyes.
Ferruginous Ducks can easily be confused with female and juvenile Tufted Ducks.
However, Tufted Ducks typically show at least a hint of a tuft or more often than not an obvious short tuft on the hindcrown.
In addition, the head shape of the Ferruginous Duck is rather peaked and the female has dark eyes whereas female Tufted Duck has distinct yellow eyes.

1st, 2nd & 3rd photo: A female (3rd shows also a male). Mila 23 area, Danube Delta, Romania, 24 May 2018
4th photo: Murighiol area, Tulcea County, Romania, 23 May 2018