Newly hatched mallards, both male and female ducklings, look very similar; they have soft, yellow fluff until about 3 weeks old when their feathers begin to grow in. Those yellow feathers begin to fade to brown.
After around two months, the male and female ducklings' feathers are fully brown, resembling the hen duck of the mallard breed.
Before winter arrives, the ducklings molt, shedding their feathers and growing new feathers that will resemble the coloration of their respective adult genders.
The following spring (during their second year) full plumage grows in. The hens will retain the brown mottled coloration, which the males will grow in those beautiful, green-metallic feathers on their heads.
To read more about distinguishing the gender of other duck breeds, click here.