All species are aquatic, with webbed feet and a broad, flattened bill. A duck is a waterfowl species related to geese and swans. There are species of ducks in most of the temperate climates of the world. The 162 species of ducks, geese and swans are found throughout the world in all manner of wetland habitats from small ditches to rugged coasts (though they generally eschew the open ocean). They range in size from the small teals (weighing 250g-300g) to the swans, which may weigh 15kg or more. Smallest Duck-Billed Dinosaur: Tethyshadros (800 Pounds) Wikimedia Commons/Tethyshadros.JPG: Ghedoghedo The second example on this list of "insular dwarfism"—that is, the tendency of animals confined to island habitats to evolve to modest proportions—the 800-pound Tethyshadros was a fraction of the size of most hadrosaurs , or duck-billed dinosaurs, which usually weighed two or three tons.