It is perhaps not surprising that birds have a reproductive system that is vastly different from that of mammals. Instead of having a penis and a vagina, most male and female birds possess an internal chamber known as the cloaca, or vent. This multi-purpose organ serves as part of their digestive and urinary systems as well as their reproductive system. It excretes faeces and urine, and in females, it is also the passage through which eggs are laid.
For most of the year, the cloaca is not visible. However, during the breeding season, when birds are feeling frisky, it swells and protrudes slightly from the bird’s body. At the same time, their reproductive organs, the testes in males and ovaries in females, also enlarge. The male’s testes produce sperm, which is stored in his cloaca until mating, while the female’s ovaries produce ova, stored deeper within her body.
While some birds engage in elaborate courtship displays and intricate mating rituals, the actual act of mating is usually over very quickly.
Once a pair bond has formed and the birds are ready to get down to business, they position themselves with the male usually balancing on top of the female. Both birds face the same direction, and the female crouches or lies down to aid the male’s balance. The male often stands on her back, placing his feet in the shallow indentations just above her wings. He then shifts his weight from side to side in a process called treading. This movement not only helps him maintain balance but also stimulates the female’s ovaries.
The female moves her tail feathers to the side, allowing the male to arch his body so their cloacas can touch. This brief contact, known as a cloacal kiss, typically lasts only a second or two. During this time, sperm is transferred to the female’s cloaca, completing the mating process. Occasionally, the birds remain in position for a while longer, engaging in several additional cloacal kisses.
For the next week or so, the birds’ sex hormone levels remain elevated, and they may mate several more times to maximise their chances of reproductive success.
After successful mating, the sperm is stored in the female’s sperm storage tubules, where it can remain viable for anywhere from a few days to several months. As eggs are released from the ovaries, or rather the single functioning ovary (see below), they are fertilised one at a time before the shell is calcified, preparing the bird to lay her eggs.
Once a bird has laid her first egg, her ovary releases the next ovum, and she will usually lay the second egg around the same time the following day. Once all the eggs have been laid, she begins incubating them, often with the help of the male. In some species, such as dotterels and phalaropes, the male takes on the full responsibility of incubation.
While embryonic birds have two ovaries, only one remains functional by the time they reach adulthood, typically the one on the left side. As a result, birds can produce only a limited number of eggs at a time.
Dinosaurs, the direct ancestors of birds, had two functioning ovaries and laid large clutches of eggs. So why did modern birds evolve to lose one ovary, seemingly reducing their chances of successfully reproducing?
Evidence suggests that the loss of one ovary was an adaptation to reduce weight, enabling more efficient flight. This would have allowed birds to escape predators more easily, explore new food sources, and access diverse habitats, offsetting the potential disadvantage of producing fewer eggs.
Adult male birds, however, have retained both testes, leading to an alternative theory for the loss of the second ovary in females. This theory suggests that the fragility of eggshells played a role. If two eggs were to pass through adjacent oviducts, they could bump against each other during flight or other activities, potentially cracking or breaking.
As always, there are exceptions. For example, kiwis have two functioning ovaries, and some raptors have been observed with bilateral ovaries.
Although 97 percent of birds lack penises, a few species do have them. These include waterfowl, tinamous, and the ratites, flightless birds such as ostriches, emus, rheas, and kiwis.
Birds’ penises differ from those of mammals in that they are an extension of the cloaca and become erect through lymph rather than blood. Usually partially feathered, they may also be covered in spines or filaments. When flaccid, they curl up inside the cloaca, and in some species, they are barely functional.
Female birds do not have vaginas in the mammalian sense. The two main components of a female bird’s reproductive system are the ovary and the oviduct. The oviduct is a long, twisted tube that can range from 25 cm to 75 cm in length, depending on the size of the bird.
The oviduct is divided into five sections: the infundibulum, magnum, isthmus, shell gland, and vagina. Unlike mammals, only one oviduct is functional. After fertilisation, the egg travels down the oviduct, where the egg white, yolk, shell, and embryo are formed.
The vagina, though not involved in egg formation, plays an important role in protecting the egg. It is where the bloom, or cuticle, forms, providing a protective layer that prevents contamination from bacteria and other microorganisms. The vagina also aids in laying the egg; it is a muscle that helps push the egg out of the hen’s body through the cloaca.
As the egg moves through the oviduct, it travels pointed end first. However, in the vagina, it rotates so that the rounded end emerges first. While this may seem counterintuitive, the rounded end is actually the strongest part of the egg. Similar to the dome structure in architecture, the rounded end can distribute pressure more evenly, providing better cushioning and reducing the likelihood of the egg breaking as it exits the bird’s body. The rounded end is also supported by an air cell just below the surface, further cushioning the egg.
Most birds have small, straight vaginas, but in female ducks, the vagina has evolved to be long and twisting, with dead-end pockets and spirals that curve in the opposite direction of the male’s penis. This adaptation helps the female duck control fertilisation in response to the male’s mating behaviour.
One Response
Oooh err missus. Is that 2 shags urm sh*gging?
Joking a part, a great article that explains all the workings but without making it too smutty, well done