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There are so many questions surrounding birds’ reproductive systems. For instance, do they mate in the air, and where are their reproductive organs? Well, bird sex organs are different from mammalian sex organs. Most male birds do not have penises, but they do have testicles.
Unlike male mammals with two equal testes, bird testicles come in all sizes and shapes. One of their testes (left) is usually bigger than the other (right). For instance, Buffleheads, a duck species, have a left testicle four times larger than the right one.
This article gives some valuable insight into a bird’s reproductive system. Keep reading to learn more.
As already mentioned, most birds have testicles but lack a penis. This begs the question: how do birds mate?
Both male and female birds have a cloaca, an internal chamber with an opening at the end. The testes and ovaries release sperm or eggs through this opening.
The cloaca opening, from both male and female birds, swells when mating and slightly protrudes outside their bodies. When the birds are active, they rub their swollen cloaca together. The male’s sperm that has been stored in the cloaca is deposited into the female’s cloaca and travels up to fertilize eggs.
Birds have sex in only one position. They cannot have sex while flying. The male bird usually perches on top of the female. The female moves her tail feathers to the side and uncovers her cloaca. The male then rubs his cloaca against the female. This act may take several attempts before they successfully copulate.
A bird’s genitals are internal, and their shape and size tends to be dictated by the space available in the internal body. Bigger birds often have bigger testes. Monogamous birds have smaller testes compared to those that undergo sperm competition.
That said, a bird is the only group of vertebrate animals that have evolved consistently into smaller testicles because they are mainly monogamous. Their polygamous counterparts have bigger testes and produce more sperm. This is necessary so the bird can have an advantage over their competition for a mate.
About 90% of birds are monogamous, meaning producing more sperms is not a high priority. According to the University of Reading, bird testes have been shrinking over time, allowing them to spend their energy on the rest of their bodies, like their plumage colorations, detailed dancing displays, and dedicated parental instincts.
The male chicken has two testes found along the rooster’s back and near the top of the kidneys. The testes are oval and light yellow. Both testicles are developed in a rooster, while a hen has only one mature ovary. A rooster keeps producing new sperm when they are mature, while a female chicken hatches with all the eggs it will ever have.
Sperm is transported from the testes through a duct called the vas deferens. Male chickens have two of these ducts and is the main area where they store sperms. When external pressure is applied at the deferens, it causes ejaculation. This is how sperm is collected for artificial insemination of hens, which is referred to as milking the rooster.
There are twisted tubes in the testicles called seminiferous tubules. The process of cell division, known as meiosis, produces the sperm in the seminiferous tubules. The sperm carry half of the chromosomes used to produce an embryo, and the ova provides the other half.
A cubic milliliter of semen produced by a rooster contains an average of 3–5 million sperm. The sperm of the fowl has a long, pointed head and a long tail when viewed under a microscope.
The testicles also produce androgen hormones, which influence the development of secondary sexual characteristics like comb growth, mating, and other male behaviors.
Penises are very rare in the world of birds. Only 3% of birds have a penis, and they are flightless birds like kiwis, emus, ostriches, and ducks. Bird penises are fundamentally different from mammal penises. They are erected by lymph (the fluid circulating through the lymphatic system) and not blood.
Bird reproduction occurs during summer and spring only—when there is plenty of food. During this period, the sizes of the testicles and ovarian follicles increase.
In male birds of all breeds, reproduction occurs under the influence of testosterone. Birds also have other effects such as increased aggressive behavior, muscle hypertrophy, and decreased fat storage that birds need for migration flights.
According to ornithologists, mating is frequently driven by the males with their violent pigment, flamboyant displays of their feathers, and aptness to initiate sexual contact.
On the other hand, in most bird species, the female makes the final decision to either accept or reject a male courting her.
The female birds have bright plumage, and they compete for the males, assuming the lead courtship role.
According to Irby J. Lovette, director of the Fuller Evolutionary Biology Program at the Cornell Lab of Ornithology, about 10% of the bird species, unlike most animals, are known to participate in interspecies mating.
A primary example is breeding between mallard ducks and black ducks found in the eastern United States. Since many hybrids born of interspecies mating die young or are incapable of passing on their genes themselves, this process is seen as evolutionarily successful.
The parent that incubates differs immensely among species. Most bird parents share incubation responsibilities. For example, Double-Crested Cormorants relieve each other frequently (every hour or less). In other birds, such as some sandpipers, doves, and pigeons, the male incubates during the day, and the female incubates at night.
In polyandrous birds where the female mates with more than one male, the male is usually the sole incubator.
Other bird species have varying patterns. One or both sexes incubate the eggs, depending on various circumstances. These circumstances can be things like the distance to food supplies and predation pressures.
Male birds have two testicles. These testicles do not hang but remain inside the body. They are found high in the body cavity behind the stomach and look like white-colored kidneys. Some birds have larger testicles than others. The left testicle in birds is normally bigger than the right one. Most birds are also monogamous and take responsibility for their young.
Featured Image Credit: Jesse Nguyen, Shutterstock
Robert’s obsession with all things optical started early in life, when his optician father would bring home prototypes for Robert to play with. Nowadays, Robert is dedicated to helping others find the right optics for their needs. His hobbies include astronomy, astrophysics, and model building. Originally from Newark, NJ, he resides in Santa Fe, New Mexico, where the nighttime skies are filled with glittering stars.
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