Duck is the common name for a number of types of bird in the family Anatidae. The ducks are divided between several different subfamilies of the Anatidae; see that article for a full listing. Ducks are mostly aquatic birds, mostly smaller than their relatives the swans and geese, and may be found in both fresh and salt water.
Ducks exploit a variety of food sources such as grasses, grains and water plants, fish, insects, and the like. The sound made by ducks is called a "quack"; a common urban legend is that quacks do not produce an echo.
The males (drakes) of northern species often have showy plumage, but this is moulted in summer to give a more female-like appearance, the "eclipse" plumage. In many species, adult males are temporarily flightless; these birds seek out protected habitat with good food supplies during this period. This moult typically precedes migration.
Some species of duck, mainly those breeding in the temperate and arctic northern hemisphere, are migratory, but others are not. Some, particularly in Australia where rainfall is patchy and erratic, are nomadic, seeking out the temporary lakes and pools that form after localised heavy rain.
Ducks have many domestic uses, being farmed for their meat, eggs, and feathers and down. In particular, eiderdown is notable for being shed by wild eider ducks rather than being plucked. In many areas, ducks of various species are also hunted for food or sport.
Ducks should not be confused with several types of unrelated birds with similar forms, such as loons or divers, grebes, gallinules, and coots.