Tuesday, December 24, 2019

Cloaca: All about how birds pee, poop, lay eggs and have sex

Why do birds poop so much? Do birds pee? Why is bird poop white? Where do birds lay eggs from? How do birds have sex?

I have decided to answer all these burning questions in one post. All these questions have one answer. It involves the cloaca, a chamber at the end of the digestive and reproductive system of reptiles, amphibians, fish, and birds.

What is a cloaca?

In birds, the bowels (digestive system), bladder and reproductive organs (urogenital system) all come together in a tubular cavity called the cloaca. The opening from the cloaca to the outside world is the anus, more often called the vent. [See references at end of article.]

This means that urine and feces from the digestive tract, and sperm and eggs from the reproductive tract, all pass out of the body through a common passage, the cloaca, and from there out the vent.

  • Where do birds pee from? The cloaca.
  • Where do birds poop from? The cloaca.
  • Where do birds lay eggs from? The cloaca.
  • How do birds have sex? The cloaca.

The word "vent" is also used by bird watchers to describe the area of a bird between and behind the legs, back to the base of the tail. The vent area feathers may be a different color than other feathers of the under parts.

Vent feathers can be useful in identification. For instance, some fall plumages of immature Orange-crowned Warblers and Tennessee Warblers are very similar. Both are rather all-over green. However, the feathers on the vent of Tennessee Warblers are white, while they are yellow-green on Orange-crowned Warblers. It is a small, but diagnostic, feature.

The vent, when discussing the feather tract between and behind the legs, are bordered by the belly in front and the undertail coverts behind. On each side of the vent feathers are the flanks.

Photo of a Brown Booby on a poop-covered buoy
Birders see only the bird; non-birders see only the bird poop.
Brown Booby. Photo by Greg Gillson.

How do birds urinate?

The kidneys of birds are rather large for their body size compared to mammals, up to 2% of body weight (Pettingill, 1970, see references at end of article). Kidneys regulate salts and liquids in the body. The kidneys also eliminate wastes from protein metabolism, in other words, urine.

Urine in birds is composed mostly of water and nitrogenous wastes, specifically uric acid. This is different from mammals whose urine has nitrogenous waste in the form of urea. The uric acid in birds is highly concentrated and nearly insoluble.

Birds do not have a bladder. Rather, urine transfers directly from the kidneys to the cloaca. The cloaca absorbs some of the water out of the urine back into the body. Between the kidneys and the cloaca, 98% of the water filtered by the kidneys is reabsorbed. This highly efficient conservation of water means that birds don't have to drink as often for urination. 

Birds lose more water through breathing than urination. It also means that the bird's urine that is voided is not liquid like a mammal. They eliminate a semi-solid pasty residue that is chalky white in color.

That's right! The white droppings birds leave behind is mostly pee, not poop, to use the common vulgar terms. Now you know why bird poop is white--it's not poop!

Where do birds poop from?

Now you know that the white excreta from birds is not so much poop as it is concentrated solids from the urine.

As we discussed, birds don't have bladders. So they don't hold their urine. This is actually a very good thing as a bladder full of urine would be heavy and make flight more difficult. It also explains why birds evacuate when taking flight--they are getting rid of excess weight.

Do birds intentionally poop on cars?

No, birds don't intentionally defecate on cars. It's just that people make it so convenient! People choose to place bird roosting sites above parking spaces! What am I talking about? Street lights, telephone wires, the edges of buildings and sidewalk trees are all placed over automobile parking spaces. Right?

All these things are wonderful bird perching structures. 

Of course, people are noisy and busy. This frightens the birds into flight. And before birds fly, they void. 

Why do birds stick around if people are always startling them? Food. People are sloppy. People spill and throw out garbage all the time. They pile garbage in bins along the street. This garbage often has some food items that some birds eat. 

Gulls, pigeons, starlings, blackbirds, house sparrows all find human food waste to eat in cities. They sit up there above your car on all these convenient perches waiting for someone to drop edible garbage. And while they sit there above your freshly washed vehicle? Digestion happens.

Bird digestive system, or... Why do birds poop so much?

The bird's digestive system starts with the mouth and proceeds to the stomach and gizzard. Partially digested food (called chime after leaving the gizzard, pronounced KIME) then continues to the small intestine. The large intestine is not obviously larger or different from the small intestine in birds. The large intestine is usually called the rectum in birds. It is actually rather short and dumps into the cloaca. The intestine is longer in seed-eating and herbivorous birds.

Birds have a high metabolism. Their body temperature normally ranges from 102-112 degrees F, depending upon species and activity. They eat nearly continuously during waking hours, if ample food is available, otherwise they rest.

Their rate of digestion is also sped up. Digestion is usually more rapid than mammals. Food items pass through a bird's digestive system in 2.5 to 12 hours. Animal food is digested faster than plant material. Pettingill (see reference at end) gives an example of a magpie digesting a mouse in 3 hours.

Birds eat 1/4 to 1/2 their weight in food each day. The Cornel Lab of Ornithology (source) says a chickadee may eat 35% of its body weight each day, a jay maybe 10%, and a hummingbird 100% in nectar plus thousands of small insects! Birds need to eat more in cold weather to keep up their high body temperature. 

What goes in must come out.

Bird poop color

Bird waste is evacuated along with the white uric acid, known as bird poop. You can often note small dark stool-shaped lumps mixed in with the larger white bird droppings. This is the actual fecal matter.

The tiny stools within the bird poop is often a dark brown or green color. But bird droppings can be stained bluish or purple if birds have been eating lots of berries, for instance. It can also be other colors depending upon diet.

The stools of Canada Geese are thick and green, composed primarily of the indigestible portions of grass that they eat. It is coated with white uric acid.

Scientists recently discovered large nesting colonies of Adelie Penguins in remote parts of the Antarctic.

How did they do so? They looked at satellite images! Adelie Penguins eat much red krill. The inedible parts of the pink shells were pooped out at their icy colony grounds and the ice was colored pink and visible from space! (Source)


The uric acid in bird excrement is rich in phosphate, potassium and nitrogen. Bird manure makes excellent fertilizer!

The word "guano" is Spanish and comes from the Quechuan language of Peru meaning dung. In Peru several seabirds return to the same islands to breed. The seabird excrement forms a thick layer on the tops of the islands.

The primary birds that produce guano off Peru are pelicans, boobies, and cormorants
Locals have been using the fertilizing properties of guano for thousands of years. In 1802 Europeans discovered how the locals were using guano in Peru. Guano then became a highly sought-after product.

By 1913 modern fertilizers were produced in Germany by the large scale artificial synthesis of ammonia. Guano became less valuable as a fertilizer then. Good thing.

The guano was collected by slaves hand mining from the small islands. The need was so strong in the United States and Europe that birds were disturbed and breeding islands destroyed. (Source)

Photo of House Sparrows copulating in a bush
Copulating House Sparrows
Photo by Greg Gillson

How do birds have sex?

Birds procreate by copulation. Males impregnate females during a "cloacal kiss." The male mounts the female and the cloacae of both the male and female are pressed together briefly. Male sperm move up the female oviduct and fertilize the egg.

The sexual union is about as quick and dull as these words suggest. However, many birds have elaborate courtship rituals for pair bonding, leading up to copulation.

Most birds have nothing resembling a penis. The testes of the male bird (well inside the body) swells immensely during the breeding season. Breeding condition of male birds is easily seen during capture and banding of wild birds for scientific study. Males in strong breeding condition have a swollen cloacal protuberance. 

Birds have sex by briefly touching their cloaca together; there is no male insertion into the female.

However, some birds such as ostriches, tinamous, some ducks, as well as chicken-like birds (including turkeys, pheasants, quail, grouse) do have a penis structure resembling that of crocodiles.

Where do birds lay eggs from?

The reproductive organs of birds are rather unusual. The gonads are markedly asymmetrical. In the females, the left ovary is functional. The right ovary is generally missing (though often present in the embryo, and present but non-functional in many birds of prey). Likewise, the male has two testes, the left is much larger, though both are functional (Audubon Encyclopedia, see references below).

After copulation the sperm may reach and fertilize the eggs within 26 minutes. In domestic hens the time between copulation and laying of fertile eggs averages about 72 hours (minimum 19.5 hours).

The female produces many ova (plural of ovum) during its life. An ovum (yolk) releases into the oviduct. It is at this point the fertilization can take place if there is any male sperm present.

About 18 minutes after fertilization the yolk passes to the magnum where the egg white (albumen) is formed. After about 3 hours in the magnum the egg is passed into the isthmus. During about an hour in the isthmus the egg receives its shell membranes. Then the egg moves to the uterus for about 20 hours where it receives the shell. The pigment color of the shell is added about 5 hours before laying. The completed egg passes through the vagina to the cloaca and is expelled into the nest.

It is all very complicated, but the egg ends up in the cloaca to be laid.


Ornithology in Laboratory and Field, 4th Edition. Olin Sewall Pettingill, Jr. 1970.

The Audubon Society Encyclopedia of North American Birds. John K. Terres. 1980.

Have you ever wondered? Do birds fly at night?


  1. Great information. Stumbled upon this article as I was searching the size and needs of these tiny birds before I decide to try raising. Not only did I not know they'd be this small I now know how the eggs and digestive system works.

  2. A very interesting article i found after researching pigeons. Thank you

    1. Thank you for visiting, and letting me know what search brought you here. It helps me plan future articles.

  3. My boys were watching themahic school bus and stumblef across an episode of egg formation. Being a chemist and taking some bio classes I wanted to refresh my mind on the topic... Thank you much, informative article.

    1. Thank you for letting me know what brought you to that article. I saw from Google that these topics were popular, but I didn't always know exactly why. So this helps me determine user intent. That way I can write better articles in the future that answer the exact question people want.

  4. Very thorough and detailed article. Helps me understand my female pigeon better. Can you write an article describing the process of what I've coined "egg turds". The ability for birds to not poop the nest when nesting (sitting on eggs) they're able to hold in their bowel movement for quite some time! It's a very fascinating process that I'm constantly searching for an in depth explanation to the phenomenon.

    1. Birds don't have the ability to "hold" bowels, at least as far as I know. I haven't heard of anything similar to what you are describing.

    2. Hens are known to sit on their nests overnight and then drop a huge nesting poop first thing in the morning. This is pretty common after laying their first and second egg.

    3. Thanks for additional clarification!

  5. Thank you for providing such excellent knowledge in the post you wrote, which includes some terrific facts that I truly appreciate.


January 2023: Thank you so much for visiting! I am working on a YouTube channel on birds and bird watching. Check it out here:



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