Although birds undergo internal fertilization, most birds do not have copulators. Only a few species, such as ostriches, ducks, geese, and swans, have copulators. The penis of ducks and geese is a spiral protrusion protruding from the ventral wall of the cloaca, with a seminal groove on its surface. When the penis is erect, the seminal groove closes at its edges to form a tube, which transports semen into the female reproductive tract. Roosters still have remnants of their copulatory apparatus, called the penis nipple, which is an oval protrusion in the middle of the bottom wall of the cloaca and anal canal. Birds are fertilized internally, regardless of whether they have copulatory apparatus or not. During mating, the cloacal openings of the male and female are close to each other.