Anas penelope bird duck fly swim water wigeon wings anas platyrhynchos beak beg begging bill feet female hopeful mallard natural nature plumage webbed Anas anatidae birds depressed body domesticated flat bill quack short legs swimming web-footed wild animal drop droplet river splash wet wild duck wing couple dimorphism England english foot Great Britain male pair riverside compete competition crowd drake ducks female duck group male duck sexual dimorphism