Asked by: Darren McIenny, Dornoch, Scotland
Mammals remove excess nitrogen from their bodies by converting it to a dilute solution of urea, stored in the bladder. Birds convert nitrogen to uric acid instead: this is metabolically more costly but saves water and weight, as it is less toxic and doesn't need to be diluted so much. Birds therefore don't have a urethra, and don't pee - all waste leaves via the anus.
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