A bird's cloaca is an important part of its anatomy. Learn about the cloaca, including how it's used and how bird banders study the cloaca for sexing birds.
What Is the Cloaca?
(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" "oh pay blah" 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 waste, expel solid waste, and 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 accumulate after a bird eats. 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, active 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 two birds position themselves so their cloacas can touch. During that brief touch, an act that is often called a "cloacal kiss," the sperm is transferred from the male's cloaca to the female's cloaca. In one mating, birds may exchange several of these "kisses" over a short time period. 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. Some female birds may store sperm for several days or weeks before eggs are fertilized. 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.
What Is the Oviduct?
The oviduct (otherwise known as the fallopian tube) is a passage through which eggs travel during reproductive processes. In birds, the oviduct is a very strong muscle that transports fertilized eggs through the mother to the outside world. After she lays her eggs, she will care for them until they hatch.
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 second or two the cloaca may be visible and can be seen as a pale peach, pinkish, or whitish bulge of skin. After excreting, the cloaca will disappear back beneath a bird's undertail covert feathers as the bird resumes a typical perch posture.
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 to 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 and examine its size to determine a bird's gender. This information is recorded along with other details about the bird being banded, such as wing measurements, weight, and feather condition, and that data can be useful for ornithologists studying bird breeding seasons and gender differences.
Also Known As
Hoffman, Ty C M et al. Cloacal evaporation: an important and previously undescribed mechanism for avian thermoregulation. The Journal of experimental biology vol. 210,Pt 5 (2007): 741-9. doi:10.1242/jeb.02705