With massive bodies and a slow laborious waddle, the Rouen breed of duck resembles its mallard ancestor in color, if not in size. Throughout most of the year, the adult males and females sport obviously different plumage, which makes it easy to differentiate between the two. The exception is during the eclipse phase when males molt all of their feathers and take on the coloring similar to a female. This phase lasts only a few weeks after breeding season.
Look for a white ring around the neck. A white ring wraps around the male's neck. The female does not have this ring.
Inspect the beak coloring. Males have a yellow or greenish beak, while females have an orange beak. During the eclipse phase, the beak color usually remains the same and can still be used to determine the gender of a duck.
Compare the plumage of the ducks. Females are mostly a golden brown with black lined patterns. Males are mostly grey with a purple–brown chest and a green back.
Look at the middle tail feathers. All the tail feathers on the female are straight. Males grow curly feathers in the middle of their tail. Some drakes as young as two months old may sport the curly tail feathers.