Among bird species with a penis are paleognathes ( tinamous and ratites) and Anatidae (ducks, geese and swans). A bird penis is different in structure from mammal penises, being an erectile expansion of the cloacal wall and being erected by lymph, not blood. It is usually partially feathered and in some species features spines and brush-like filaments, and in flaccid state curls up inside the cloaca. The lake duck (also called Argentine blue-bill) has the largest penis in relation to body size of all vertebrates while usually about half the body size (20 cm), a specimen with a penis 42.5 cm long is documented. While most male birds have no external genitalia, male waterfowl ( Anatidae) have a phallus. Most birds mate with the males balancing on top of the females and touching cloacas in a 'cloacal kiss' this makes forceful insemination very difficult.