(noun) The cloaca is the single posterior opening for a bird's digestive, urinary and reproductive tracts and is used to expel feces and lay eggs. The cloaca is found on the rear of the body under the base of the tail, covered by feathers on the extreme lower abdomen. The cloaca and the area of the body where it is located, as well as the physical slit opening itself, is also frequently called the vent.
(rhymes with "tow bay rah" or "go play ma")
About the Cloaca
A bird's cloaca is the end of several internal systems, including the digestive, excretory and reproductive tracts. Instead of having separate openings to expel liquid and solid waste as well as lay eggs, the cloaca serves all those functions as needed. Inside the cloaca there are several skin and muscle folds that subdivide the chamber depending on how it is needed to be used.
The cloaca is at the end of a bird's digestive tract, where both feces and urine will accumulate. Depending on the bird, these wastes may be stored in different chambers of the cloaca, generally with solid material stored in the innermost chamber as nutrient absorption continues. Liquid and solid waste is mixed together and expelled simultaneously after a bird's digestion is complete.
Reproduction is more complicated, but both male and female birds have a cloaca.
When birds are ready to mate, the male stores healthy sperm in his cloaca. As a bird gets ready to mate, hormonal changes cause the cloaca to swell and protrude slightly from the body. Mating only takes a few seconds for birds, when they position themselves so their cloacas can touch to transfer sperm from the male to the female, an act that is often called a "cloacal kiss." The sperm is then stored in the female's reproductive system until it fertilizes an egg, generally in the oviduct after passing from the female's cloaca and through her vagina.
After fertilization, the albumen and shell parts of the egg are deposited, and the egg is passed down through the vagina and out the cloaca as it is laid.
It is also possible that some bird species may use the cloaca as part of temperature regulation. If the cloaca is swollen and protruding in hot temperatures, the bird's body can be cooled through evaporative cooling, similar to a dog panting on a hot day. Very little study has been done on this process, however, and more research is needed to firmly conclude whether or not the cloaca has a significant temperature regulation effect for any bird species.
In addition to birds, amphibians, reptiles, sharks and rays have a cloaca, the same type of common opening for the digestive, excretory and reproductive systems.
Bird Identification With the Cloaca
A bird's cloaca is usually covered with feathers and not able to be seen during casual observation. When a bird expels feces, it will bend slightly forward and raise its tail as it excretes. For a fraction of a second the cloaca may be visible and can be seen as a pale pinkish or whitish bulge of skin. After excreting, the cloaca will disappear back beneath a bird's undertail covert feathers.
In rare instances, injury or infection may cause the cloaca to swell and be more visible. There are no distinct markings on the cloaca, however, and it cannot be used for basic bird identification. At times, a bird may have fecal matter dangling from its cloaca temporarily, but the bird will shake its rump and tail and otherwise dislodge any clinging material.
There are times, however, when the cloaca can be used to tell the difference between male and female birds. During the breeding season, the cloacal area swells and the tissues protrude slightly outside the body to make breeding easier. Male birds show more swelling, and bird banders will gently blow on a bird's vent to expose the cloaca to determine a bird's gender. This information is recorded along with other details about the bird being banded, and that data can be useful for ornithologists studying bird breeding seasons.
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