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Simple: because they don’t have a bladder to store urine. Whenever a bird drinks some liquid, it goes straight to the intestine, where it is absorbed; then it passes into the blood and reaches the kidneys to be purified. The impurities left over from this process – mainly a substance known as urate – are carried by a canal directly from the kidneys to the intestines, where they are deposited along with the feces and expelled through the cloaca, in the same opening as the genitals. “That’s why you can see a whitish part in the feces of most birds, formed by urate”, says ornithologist Werner Bockerman, from the Zoological Park Foundation of São Paulo.