The white in bird droppings is predominantly uric acid excreted by the kidneys. Unlike mammals, birds don’t excrete the products of protein breakdown as soluble urea, but as uric acid. This is only marginally soluble, so is seen as a white paste. This comes from the kidney into the cloaca – the single orifice for excretion, urination, egg-laying and mating – where the faecal waste also empties.
This means bird droppings are a mix of faeces and uric acid. However, the faeces tend to be produced first
Originally posted by Metabunk.org
