There are no distinguishing features on Muscovy ducklings to determine sex. As with most domestic fowl, the sex of a Muscovy duck younger than four weeks of age can be determined only by a process called "venting." As the ducks grow, however, the features that distinguish the males (drakes) from the females (hens) gradually become apparent. By the time the ducks are approaching adulthood, the differences between the males and females can be determined by observation, even by the untrained eye.
Observe the ducklings from four to six weeks of age. The young drakes (males) will begin to gain size over the young hens (females).
Observe the ducklings from six to 10 weeks of age. The young females will develop their flight feathers before the young males. The young males will have thicker legs, a wider stance and larger feet than the young females.
Observe the young ducks after 10 weeks of age. The male ducks will develop more pronounced caruncling (fleshy, bulbous facial mask) than the females.
Listen to and observe the ducks. Both sexes make a trill type of sound, but females are more vocal than males. Male Muscovy ducks hiss more than females, accompanied by a tail wagging posture and feather fluffing.
Venting should be left to professional venters. The process of venting a hatchling Muscovy duck can permanently damage the duck if done improperly.