You, me, cats, dogs, horses, rabbits, deer, elephants and lots of other animals all make brown poop (most of the time, anyway). What makes birds veer from the standard color palette and produce black and white poo?
Birds’ digestive systems and naughty bits don’t work exactly like ours or most other animals'. Instead of pooping and peeing separately, they basically do it all in one weird mess. Their kidneys extract nitrogenous wastes from the bloodstream like other animals’ do, but instead of releasing it as urea dissolved in urine, birds excrete it in the form of uric acid. It comes out as a white sort of goo because of the biochemical reactions that happen to process the waste so it can be safely excreted with minimal water loss.
The way it's excreted is also a little weird compared to the rest of us. Most bird species don’t have “traditional” penises and vaginas (though there are some bizarre exceptions). Instead, both sexes have a cloaca — an all-purpose entrance and exit for the intestinal, reproductive, and urinary tracts. It’s used to expel waste, lay eggs and have sex (which, for birds, happens in the form of a “cloacal kiss”). This orificial multitasking explains the dark bullseye that’s often in the center of the white acid waste. That’s the actual “poop” part, or stool. Because the acid and poop are expelled at the same time from the same opening, but from two different bodily systems, they don’t have much time to blend, and you get a bird dropping with two distinct parts that looks like a poor man’s Rorschach test.
The acidic attributes of bird poop are a detriment to your car’s paint job, but it's highly sought after for what it can do to your face. Maybe not your face, but certain celebrities go nuts for bird poop facials, where Japanese Nightingale poop is mixed with rice bran and water and used to exfoliate the skin.