Why is bird poop white?

Bird droppings differ from mammal waste, consisting of a brown solid part and a white substance. The brown part is undigested food, while the white is uric acid, a less toxic waste product. Birds convert nitrogen waste into uric acid instead of urea, allowing them to conserve water during flight. Their kidneys excrete this waste in solid form, which is expelled through the cloaca. Interestingly, some species, like the Cyclosa ginnaga spider, mimic bird droppings for camouflage. Historically, bird droppings, or guano, were valuable as fertilizer, leading to protective measures for seabirds in ancient cultures.