Female ducks are called either hens or ducks, while the males are called drakes, and babies are called ducklings. In many duck species, females have less colorful feather patterns than males, making them easier to identify.
Another distinguishing feature between hens and drakes are the sounds they make. Only hens produce the "quack-quack" noise frequently associated with ducks. Drakes make a softer sound that's usually more of a whistle or a whisper.
Ducks with tail feathers that curl upward are almost always drakes; exceptions can occur if there is no male and a hen fills the drake roll. Immediately following the mating season both drakes and hens begin molting and remain flightless for a couple months. During this time the two sexes are difficult to distinguish visually, and sound is the best indicator.