There are more than 150 species of ducks, geese, and swans in the family Anatidae and the Order Anseriformes.  The only other living family in this order consists of three species of Screamers in the family Anhimidae known from southern South America.  The birds of the family Anatidae have wingspans which range from a foot and a half to just under 7 feet.  Compared to most other birds, their heads are large, their necks are long, and their tails are short. The bills of these birds are usually blunt and wide. 

     Ducks, geese, and swans possess waterproof feathers and a thick layer of down for insulation.   These birds are unusual in that they molt their flight feathers all at once since most birds gradually replace their flight feathers. This sudden molting leaves them flightless for a period of three weeks to a month during the time when the young are still fledglings.  The down which covers the young insulates them and makes them more buoyant.  Most ducks make concealed nests on the ground near water but some, such as wood ducks, mergansers, buffleheads, and goldeneyes, nest in tree cavities.  Ground-nesting species typically suffer greater predation on their young than tree-nesting species (Sibley, 2001).

     Differences in the bill structure allow individual species to specialize on different types of food.  The bills of Canada geese are adept at cutting grass.  Green-winged teals can feed on tiny organisms because of small comb-like structures.  Northern shovelers have bills adapted to hunting invertebrates.  The serrated, narrow bills of mergansers allow them to better grip the fish and invertebrates of their diet (Sibley, 2001).

      The family Anatidae is divided into three subfamilies: Anseranatinae which contains a  single genus of Australian goose, Anserinae which contains several genera of swans and true geese, Dendrocygninae which contains the whistling ducks, and Anatinae which contains the majority of ducks.  Most of the subfamily Anserinae are large.

     Although increased conservation has boosted population sizes of many species since their low numbers of the 1930s, some populations continue to decline such as black ducks and pintails.  The loss of wetlands is a major challenge to these aquatic birds (Sibley, 2001).