While protein is essential in the diet of all animals, it can have side effects that make it a double-edged sword.

Protein consists of many amino acids, each containing a nitrogen group. Following digestion, the amino acids are brought to the liver, where they are used in a variety of important functions. Ultimately, they need to be excreted from the body, and therein lies the problem.

Fish excrete the nitrogen as ammonia, a compound that is quite toxic. Because fish spend their lives swimming in an aquatic environment, however, they can excrete the ammonia with their urine directly into their surroundings — constantly.

Humans and other mammals metabolize the nitrogen to urea, which is less toxic than ammonia and can be stored in diluted form in a urinary bladder until it is convenient to pass. A bladder that stores urine increases body weight automatically. It also increases the need to drink to compensate for water lost with the urine.

Neither condition is in the best interest of birds, which have evolved weight-loss adaptations for flight and methods for water conservation. And less time spent drinking helps birds increase their chances of avoiding predation.

To avoid the toxicity of ammonia and the extra weight of a urinary bladder, birds, in their kidneys, metabolize nitrogen waste to uric acid, a white, chalky precipitate that is not only non-toxic but also almost dry. This always prompts disbelief from people whose car windshields have been decorated with droppings that are anything but dry, but it’s true. To understand how, we need to consider a structure called the cloaca.

The cloaca

Located at the end of the large intestine, the cloaca is a compartment that opens directly to the outside by way of a vent, or anal opening.

In addition to receiving egested waste from the intestine (which accounts for the splat on your windshield), the cloaca receives tubes known as ureters from the kidneys, and either a uterus from the ovary (if female) or vas deferens from the testes (if male). Not surprisingly, one definition of the word cloaca is “common sewer.”

If you have ever looked inside a songbird nest after the young have fledged, you may have noticed how clean it is. And it is clean despite the fact that the nestlings usually defecate after each feeding. Because adults feed their young nearly continuously, you would expect a huge pile of fecal waste, but it doesn’t build up — thanks to the cloaca.

Fecal sacs

In newly hatched songbirds and other birds, the cells lining the cloaca secrete a gelatinous material that surrounds the incoming waste. The result is a soft, neatly packaged capsule called a fecal sac. After a nestling has been fed by its parents, it will typically lower its head, raise its tail, and eject it.

The adult remains by the nest immediately after feeding, grabs the fecal sac in its bill, and either swallows it or flies away, dropping the sac some distance from the nest. In either case, nest sanitation is maintained. In forming the waste capsule, the cloaca performs an important function: It helps to reduce diseases, parasites, and odors that might attract predators to the nest.

Some years ago, I observed nesting Hermit Thrushes from a blind. Once hatching had occurred, the parents fed the chicks with amazing frequency. I observed hundreds of feeding trips during which the adults waited to remove a fecal sac.

Only once did an adult leave before the sac appeared. On this occasion, the nestling waited a bit too long before ejecting a fecal sac and it fell harmlessly to the floor of the nest, where it remained throughout the nesting period.

Other sanitation methods

It’s possible to keep nests tidy without using fecal sacs. Kingfishers lay their eggs in tunnels in banks or cliffs. When their young need to void, they walk to the tunnel’s opening, turn around, and eject their waste outward. The same is true for Bank Swallows.

Nestling hawks and eagles do not produce fecal sacs but avoid sullying their nest by backing to the rim and squirting a stream of whitish waste over the side. Whitewash spotted on the ground near a tree or on the base of a tree is a good indicator of a nest in the canopy.

Falcons, which typically nest on cliffs, simply drop their feces and urine mixture over the edge, leaving much white streaking on the cliff below. Digging through the ground at the base of a cliff can reveal layers of “annual” fecal accumulation that can be dated back centuries. Some Peregrine Falcon nest sites on cliffs in Great Britain go back to the Middle Ages, and Gyrfalcon nests in Greenland are even older. (See sidebar.)

Birds’ clever sanitary engineering — excreting non-toxic uric acid, reducing the need to take in water, and forming fecal sacs — has led to their amazing success.

 

This article from Eldon Greij’s column “Amazing Birds” appeared in the July/August 2014 issue of BirdWatching.

 

New to BirdWatching?

Sign up for our free e-newsletter to receive news, photos of birds, attracting and ID tips, descriptions of birding hotspots, and more delivered to your inbox every other week. Sign up now

See the contents of our current issue

How to subscribe to BirdWatching

  Originally Published July 8, 2014