Why is bird poop white again?
Unlike mammals, birds don’t urinate. Their kidneys extract nitrogenous wastes from the bloodstream, but instead of excreting it as urea dissolved in urine as we do, they excrete it in the form of uric acid.
Uric acid has a very low solubility in water, so it emerges as a white paste. This material, as well as the output of the intestines, emerges from the bird’s cloaca. The cloaca is a multi-purpose hole for birds: their wastes come out of it, they have sex by putting their cloacas together, and females lay eggs out of it.
Brown excrement. The color comes mainly from bilirubin, a pigment that arises from the breakdown of red blood cells in the liver and bone marrow. The actual metabolic pathway of bilirubin and its byproducts in the body is very complicated, so we will simply say that a lot of it ends up in the intestine, where it is further modified by bacterial action. But the color itself comes from iron. Iron in hemoglobin in red blood cells gives blood its red color, and iron in the waste product bilirubin gives rise to its brown color.