Everyone poops, but birds do it a little differently than we do. Next time you are washing bird poop off your car, or laughing at eagle poop shoots, take time to consider an eagle’s whole pooping process!
The basic chain of events goes something like this. An eagle catches a fish and eats it. The bits of fish move from the eagle’s esophagus into an expandable storage pouch called the crop, which allows birds to gorge food much faster than they can digest it. From the crop, food trickles into the true stomach and the gizzard, a muscular organ that grinds the fish bits into small pieces. Nutrients are absorbed in the eagle’s small intestine and put to work in its cells, where they assist in growing new cells, maintaining existing cells, and manufacturing the products an eagle’s body needs to stay healthy.
Waste food products are discharged via defecation, aka pooping. Waste created by metabolic processes is discharged via excretion, which includes urination. Humans turn waste nitrogen into urea, which we dissolve in water to form urine. But this isn’t an option for birds. Embryonic birds can’t dissolve waste in water and expel it through an eggshell, and altricial nestling birds can’t access water in the nest. Many birds live in or spend long amounts of time in areas that don’t have much drinking water – think of migrants like the Arctic Tern, or resident birds like the Emperor Penguin. And water is also heavy. One ounce of water weighs 28 grams. That’s a lot of weight for a bird like the ruby-crowned kinglet, which weighs just five to ten grams. Adult humans are are supposed to drink eight 16-ounce glasses of water a day. That equals .987 pounds – or about 45 ruby-crowned kinglets!
So how do birds get rid of nitrogen? They bind it with uric acid, which uses much less water than our nitrogen-to-urea-to-urine process. Excretion via urea usually creates transparent clear to yellow pee (see this link for more information about the other colors pee can come in). In birds, excretion via uric acid creates the pasty white part of bird ‘poop’. It takes more energy to synthesize uric acid, but that isn’t as important as removing the need for water. Minimizing water requirements helps birds stay light!
Feces make up the dark part of bird ‘poop’ and are formed in a bird’s body much like they are formed in ours. However, birds are monotremes. Their intestinal, urinary, and reproductive systems all terminate in a single posterior orifice called the cloaca, derived from the Latin word for sewer. Pasty white bird urine and feces aren’t the same thing, but they both exit from the same place, sometimes at the same time, and sometimes quite explosively!
It is interesting to speculate that birds developed as they did in part because they don’t need water for excretion. While large birds might be able to carry the extra weight, it seems unlikely we would have small birds given that water is heavy. We probably wouldn’t have birds that migrate across or live in extremely dry places like the antarctic, the world’s deserts, or the world’s oceans. And altricial birds would have had to have solved the problem of getting water to nestlings or they wouldn’t have survived. I will try to be more grateful for bird excrement the next time I am washing it off my car!
Things that helped me learn and write about this article: