The avian male reproductive tract is very unique and different from that of humans and mammals. Male birds have two functional bean-shaped testes located within the body, just above the kidneys. This is in contrast to many female birds in which the right ovary regresses, leaving just one functional left ovary. The testes grow in size as the bird reaches sexual maturity. In seasonal breeders, the testes enlarge during the breeding season and often change color – from yellow to white in most birds. Immature or inactive testes are yellow in color due to the acumulation of lipids. During the onset of breeding, the testes change to a white color due to the increase in the size of the siminiferus tubules. The testes are covered by a very thin tunic. The bulk of the testis is composed of numerous convoluted seminiferous tubules. Sperm cells are formed in the tubules. Sperm formation occurs more rapidly in birds as compared to mammals.

The testes also produce the male hormone testosterone. This hormone is very important in stimulating growth of the male reproductive tract, the development of secondary sexual characteristics such as courtship behavior and aggression and, in chickens, the combs and wattles.

Once the sperm leave the testes, they travel through a small tube, known as the epididymis . The avian epdidymis is very small in the bird and is not divided into three parts, the head, the body and the tail as in mammals.

Sperm then enter the ductus deferens a long narrow tube that travels next to the ureter and enters the cloaca. The ductus is densely packed with sperm during the breeding season. It takes from one to four days for the sperm to travel from the testes to the end of the ductus. Sperm undergo maturation in the male reproductive tract.

In psittacines (parrots), the main storage place for the sperm is the ductus deferens. In passerines (finches, canaries, etc.) sperm is stored in the “seminal glomus”, a convoluted structure at the end of the ductus. The right and left seminal glomis form a projection in the cloaca known as a cloaca promatory which can be used to sex passerines.

The spermatazoon is composed of an acrosome, a head and a tail. The acrosome contains an enzyme which enables the sperm to penetrate the egg.

Psittacines do not have a phallus. They mate by joining their cloacas, with the male ejaculating sperm into the female’s cloaca. Some species of birds – such as ostriches, ducks, geese, swans, chickens and turkeys do have a phallus – but it is different from the mammalian phallus.

Castration of male birds has been considered by some as a way of curbing aggressive behavior. The surgery is more complex than in mammals, since the avian testes are located within the body – unlike mammalian testes which are located outside the body. Two incisions are necessary to remove the avian testes – one on the right and one on the left. This surgery is a difficult surgery and is not recommended as a means to treat aggressive behavior in birds.