As animals digest proteins, they produce ammonia. That ammonia is toxic, so most animals convert it into something else. Mammals convert it to urea, which is less toxic. That urea still has to be diluted with lots of water, which they get rid of as urine. Birds and reptiles have a different strategy. They convert the ammonia into uric acid. Uric acid is much less toxic than urea, so it does not have to be diluted with water. As a result, bird and reptile droppings usually contain a lot of white uric acid.