Duck is the common name for a large number of species in the waterfowl family Anatidae, which also includes swans and geese. Ducks are divided among several subfamilies in the family Anatidae; they do not represent a monophyletic group (the group of all descendants of a single common ancestral species) but a form taxon, since swans and geese are not considered ducks. Ducks are mostly aquatic birds, mostly smaller than the swans and geese, and may be found in both fresh water and sea water.
Ducks are sometimes confused with several types of unrelated water birds with similar forms, such as loons or divers, grebes, gallinules, and coots.
Ducks eat a variety of food sources such as grasses, aquatic plants, fish, insects, small amphibians, worms, and small molluscs.
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 and to hold slippery food items.
Diving ducks and sea ducks forage deep underwater. To be able to submerge more easily, the diving ducks are heavier than dabbling ducks, and therefore have more difficulty taking off to fly.
A few specialized species such as the mergansers are adapted to catch and swallow large fish.
The others have the characteristic wide flat beak adapted to dredging-type jobs such as pulling up waterweed, pulling worms and small molluscs out of mud, searching for insect larvae, and bulk jobs such as dredging out, holding, turning head first, and swallowing a squirming frog. To avoid injury when digging into sediment it has no cere, but the nostrils come out through hard horn.
The Guardian (British newspaper) published an article advising that ducks should not be fed with bread because it damages the health of the ducks and pollutes waterways.