|The brown bird on the left is the female; the brightly colored one on the right is the male, called a drake. This is the most common duck in the world, the Mallard (Anas platyrhynchos)|
Ducks are birds in the family Anatidae. Ducks are closely related to swans and geese, which are in the same family. Ducks are not a monophyletic group. They are a 'form taxon', since swans and geese (who are in the same family) are not called ducks. The main difference is that ducks have shorter necks, and are smaller.
Other swimming and diving birds, like grebes and loons, are sometimes called ducks, but they are not. A baby duck is called a duckling, and a male duck is called a drake. Most ducks are aquatic birds. They can be found in both saltwater and fresh water.
Ducks lay eggs once a year and are omnivorous, eating aquatic plants and tiny animals. Dabbling ducks feed on the surface of water or on land, or as deep as they can reach by up-ending without completely submerging. Along the edge of the beak there is a comb-like structure called a pecten. This strains the water squirting from the side of the beak and traps any food. The pecten is also used to preen feathers. Diving ducks dive deep to get their food.
Many ducks are migratory. This means that they spend the summer months in a different place than the winter months. Ducks show a cosmopolitan distribution, they can be found all over the world, except for Antarctica. Some duck species live on the South Georgia and Auckland Islands, which are subantarctic. Many species have established themselves on remote islands, such as Kerguelen or Hawaii.
Some ducks are bred and kept by humans. They are not wild ducks. They are kept to provide food (meat and eggs), or to use their feathers for pillows and other items in the house. Especially in Asia, many people like to eat duck.
Ducks are sometimes kept as pets. They are often kept by groups of people on public ponds for their beauty and calming nature. People commonly feed ducks in ponds stale bread, thinking that the ducks will like to have something to eat. However bread is not healthy for ducks and can kill them.