To start with the basics, both the excretory anatomy and the consistency of bird droppings are different from those of most mammals. Birds have one opening, a cloaca, which serves as their intestinal, reproductive and urinary tract. Everything from mating and laying eggs to voiding waste takes place via the cloaca.
Instead of releasing waste as urea dissolved in urine, as we do, birds excrete it in the form of uric acid, which is the white liquid we associate with bird droppings. Why is it white instead of brown? This is due to the biochemical reactions that take place in processing the waste so it can be safely excreted with minimal water loss.
If you look carefully at a bird’s droppings, you’ll usually find a small dark solid blob amongst all the white uric acid. Both acid and solid waste are evacuated from the same opening at the same time. Because they come from two different bodily systems, they don’t have much time to blend.
Most people are not great fans of bird droppings due to the damage (and mess) they can cause. However, some ingenious soul took advantage of this property and concocted a skin exfoliator for sale that consists of nightingale droppings, water and rice bran.
(This post is dedicated to my sister who didn’t quite believe I would actually write a post about this subject much less use this Snowy Owl photograph.)