Duck is the common name for numerous species of waterfowl in the family Anatidae. Ducks are generally smaller and shorter-necked than swans and geese, which are members of the same family. Divided among several subfamilies, they are a form taxon; they do not represent a monophyletic group (the group of all descendants of a single common ancestral species), since swans and geese are not considered ducks. Ducks are mostly aquatic birds, 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. Etymology The word duck comes from Old English 'diver', a derivative of the verb 'to duck, bend down low as if to get under something, or dive', because of the way many species in the dabbling duck group feed by upending; compare with Dutch and German 'to dive'. This word replaced Old English / 'duck', possibly to avoid confusion with other words, such as 'end' with similar forms. Other Germanic languages still have similar words for duck, for example, Dutch , German and Norwegian . The word / was inherited from Proto-Indo-European; cf. Latin anas "duck", Lithuanian 'duck', Ancient Greek / (/) 'duck', and Sanskrit 'water bird', among others. A duckling is a young duck in downy plumage or baby duck, but in the food trade a young domestic duck which has just reached adult size and bulk and its meat is still fully tender, is sometimes labelled as a duckling. A male is called a drake and the female is called a duck, or in ornithology a hen. Taxonomy All ducks belong to the biological order Anseriformes, a group that contains the ducks, geese and swans, as well as the screamers, and the magpie goose. All except the screamers belong to the biological family Anatidae. Within the family, ducks are split into a variety of subfamilies and 'tribes'. The number and composition of these subfamilies and tribes is the cause of considerable disagreement among taxonomists. Some base their decisions on morphological characteristics, others on shared behaviours or genetic studies. The number of suggested subfamilies containing ducks ranges from two to five. The significant level of hybridisation that occurs among wild ducks complicates efforts to tease apart the relationships between various species. In most modern classifications, the so-called 'true ducks' belong to the subfamily Anatinae,