Anseriformes in a large Order of mostly web-footed aquatic birds.  There are about 180 species in 3 families,  Anhimidae (Screamers),  Anseranatidae (the Magpie Goose) and Anatidae with over 170 species of ducks, geese and swans. As an adaptation to mating in the water, with the exception of Screamers, males have penises.  They are only one of only a few bird groups to survive the extinction of dinosaurs to the present.  Taxonomists differ in assigning some species assignments and whether to treat groups as subfamilies or tribes.