Not every male animal has a penis—and bird versions differ
A penis is a male intromittent organ for delivering sperm, found in many vertebrates and invertebrates yet absent in plenty of species and not always homologous. Placental mammals also route urine through it. Ancestral amniotes likely had one; most male birds instead use a cloacal kiss.
Biologists use the term penis for the male organ that delivers sperm into a female or hermaphrodite during mating. Such organs turn up in both vertebrates and invertebrates, yet many species lack one, and the organs that do exist did not all evolve from a common structure. Not every sperm-transfer organ earns the name either: most cephalopods use a modified arm called a hectocotylus, male spiders use their pedipalps, and some reptiles have paired hemipenes with their own terminology.
In most animals that have one, the organ's only real job is getting sperm to the female. Placental mammals are the exception, because their penis also carries the end of the urethra, so it handles urine as well as semen. The last common ancestor of mammals, birds and reptiles probably had a penis, and researchers suggest those of living amniotes share a single developmental origin.
Most male birds, roosters and turkeys among them, have only a cloaca, and mating is a brief cloacal kiss with the male balanced on the female. The exceptions are the paleognaths, meaning tinamous and ratites, and the waterfowl family of ducks, geese and swans. A bird's version is an expansion of the cloacal wall stiffened by lymph rather than blood, often partly feathered and sometimes spiny, and it curls inside the cloaca when at rest.
Waterfowl show an evolutionary arms race. The male's organ everts in a coil that allows mating without the female's cooperation, and females have evolved dead-end sacs and oppositely coiled passages that make that harder. The lake duck holds the vertebrate record relative to body size: fully extended it can match the bird's own length, though half is more typical, and its bristled tip may scrub out rivals' sperm.
Source: Penis
