bird 'poo' is actually made up of two constituents, solid food waste (usually brown or black, depending on the species diet) and the white part which is actually equivalent to our urine. birds (and reptiles, whom the birds evolved from) have only one excretory outlet compared to the two found in mammals; a multipurpose opening known as the cloaca. the cloaca excretes both forms of waste and also serves as the reproductive opening, allowing exchange of genetic material and subsequent egglaying. 

bird urine appears white because birds excrete their nitrogenous waste as uric acid, rather than urea and ammonia as in mammals. the reason for this is that uric acid can be excreted as a non toxic paste, allowing greater water retention thereby reducing the amount of water needed. this is crucial in flying birds for whom economy of mass is very important. it is also important because birds and reptiles lay eggs. the egg membrane allows exchange of gases but not solid waste, so the smaller the quantity of waste, the better. the ability to excrete a small amount of non toxic solid waste, rather than a large quantity of diluted waste as produced by mammals is imperative.