any of various species of relatively small, short-necked, large-billed waterfowl. In true ducks—i.e., those classified in the subfamily Anatinae—the legs are placed rearward, as in swans (swan), rather than forward, as in geese (goose). The result is a distinctive waddling gait. Most true ducks, including a few inaccurately called geese (e.g., sheldgeese (sheldgoose)) by reason of size and build, also differ from swans and true geese in the following characteristics: males (drakes) and females (hens) exhibit some degree of differentiation in plumage and in call, males molt twice annually, females lay large clutches of smooth-shelled rather than rough-shelled eggs, and both sexes have overlapping scales on the skin of the leg.

All true ducks, except those in the shelduck group and sea ducks, mature in the first year and pair only for the season—unlike the late-maturing, life-mating true geese and swans. Ducks are generally divided into three major groups, dabbling (dabbling duck) (shallow-water), diving, and perching ducks, based on their characteristic behaviours. The mallard, a typical dabbling duck, is one of the most popular game birds and is the ancestor of most domestic ducks (see poultry). pintails, teals, shovelers, and wigeons are also dabbling ducks, in addition to the gadwall and black duck. Perching ducks (perching duck) such as the muscovy, wood duck, and mandarin duck have long claws and are the most arboreal of ducks, often roosting in trees. The diving ducks (diving duck) include the greatest number of marine species, such as eiders and scoters, but they also include primarily freshwater species such as mergansers, ring-necked ducks, and pochards, including scaups and the canvasback. The redhead, goldeneye, and bufflehead are diving ducks that live in fresh and saltwater, depending on the season. Members of the stifftail group, typified by the blue-billed ruddy duck (Oxyura jamaicensis), are highly aquatic diving ducks characterized by legs set far toward the rear of the body. The whistling ducks (genus Dendrocygna), also called tree ducks, are not true ducks but are more closely related to the geese and swans. The freckled duck and torrent duck, as well as the shelduck and steamer duck, are other anomalous species. All ducks, geese, and swans belong to the family Anatidae. For a more detailed classification, see anseriform.