To feather-sex ducks, or determine the gender of ducks on the basis of their feathers, use the following guideline: A male duck, or drake, displays nuptial plumage—a curled feather on the tail—prior to and during the breeding season. Muscovy breed females do not have the curved sex feathers.
Figure 1. Male mallard duck with nuptial plumage. Note the curled feather on the tail. Source: Jacquie Jacob, University of Kentucky
Figure 2. Female mallard duck. Note the lack of the curled tailed feathers found on the male. Source: Jacquie Jacob, University of Kentucky