Saturday, November 07, 2015

Why do birds have white poop?

This video of Mom voiding made me think about bird poop. I used to think that defecation and excretion were two words for same thing. Poop is excrement, right? But I was wrong. Defecation is the discharging of waste that was once food. Excretion is the discharging of waste created by metabolic processes - biochemical reactions that take place within living cells to maintain life. We are all familiar with the basic chain of events, which 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 from into the true stomach and the gizzard, a muscular stomach that uses grit to grind the fish bits into even smaller 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.

Like food digestion, cellular processes create waste products that need to be removed from the body. In particular, humans get rid of excess nitrogen by combining it with other molecules to form urea. However, water is required to dissolve urea in urine. That isn't a problem for us, but it is for birds. Embryonic birds can't dissolve waste in water and expel it through an eggshell. Altricial nestling birds can't access water in the nest. Many birds (and presumably bird ancestors) live in or spend long amounts of time in areas that don't have adequate 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. Us adult human types are are supposed to drink eight 16-ounce glasses of water a day, which equals .987 pounds - far heavier than the transmitters we put on D1, D14, Indy, and Four.

So what do birds do to get rid of nitrogen? They bind it with uric acid. In humans, 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 water for animals that live like birds do.

 So how about the black part of bird poop? It is feces, which are removed from a bird's body much like they are removed from ours. However, birds are monotremes - that is, 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 from my car.

Things that helped me learn and write about this article: