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.
Read more:
Subscribe to BBC Focus magazine for fascinating new Q&As every month and follow @sciencefocusQA on Twitter for your daily dose of fun science facts.
