There are many ways to sex a duck. Besides listening to their call, visual cues are the easiest way to tell boy ducks apart from girl ducks. In most breeds the males are larger and have more colorful feathers and markings. Even on breeds with little to no sexual dimorphism, the males tend to have curly tail feathers. Another clue is that female ducks, once they reach egg-laying age, develop freckles on their bills, while male ducks have bare bills throughout their lives.
Some ducks run fast! Indian Runner ducks are originally from Southeast Asia, where they were herded from the house to the rice paddy each day. They had to run fast to keep up with their human herders. With tall, slender, upright bodies, they look like a cross between a mallard and a penguin. Indian Runner ducks are known for their ability to hunt down bugs, slugs and snails and were traditionally employed to perform pest control in the rice paddies. Today they are often used for backyard garden pest control.
Ducks with funny hairdos have a genetic disadvantage. Crested ducks, which are popular in duck shows, have an orb-shaped clump of feathers on the top of their heads that, depending on the breed, can resemble the hairdo of George Washington, a rock star with a mohawk or an old woman with a bun. Unfortunately (for the ducks) the fancy hair is a result of inbreeding and is often associated with other, more problematic deformities. If you breed a crested duck with a crested duck, many of the offspring die before they hatch, while others emerge with twisted necks or other problems that leave them disabled.
Ducks turn white with age. White ducks are white their whole lives, but dark-colored ducks slowly develop white patches in their feathers as they age, just like human hair. Eventually, when they are very old, they may become entirely white.
Ducks have very unusual sexual organs. Most birds do not have penises or vaginal canals at all, mating instead with a ‘cloacal kiss’. But ducks are one of the few birds whose genitalia (vaguely) resembles our own. The subject has only recently been studied by scientists, but it turns out that males have springy corkscrew-shaped penises up to 16 inches in length and females have a corresponding spiral-shaped vaginal canal.