Young mallards are generally possible to sex by field characters already before they have become independent from their mother (see below). Adult birds are always easy to sex according to bill patterns, although genetically depleted feral birds (and perhaps other?) may deviate from the normal patterns. Males in eclipse plumage may appear superficially similar to females but differentiate from females by blacker crown and more greyish face and neck.
The young males soon develop a greenish-yellow bill, which often appear plain with a (less extensive) dark area on the central upper ridge.
Females (young and adult) have an orange bill with a variable blackish pattern often covering a large part of the upper mandible.
1cy male September. Note greenish-yellow bill. [90A88313]
1cy female, September. Bill showing an orange basic colour. Note that many females show traces of green colour in the crown feathers. [90A87350]