It is a common misconception that the white droppings produced by birds are feces.
In fact, they are urine. Birds excrete uric acid rather than urea because it is an insoluble solid. This way they avoid wasting water when urinating, just one of their adaptations for a good power-to-weight ratio.
The white material that comprises the droppings of birds, and indeed many reptiles, is their urine.
The more primitive vertebrates excrete toxic nitrogenous waste relatively directly, having masses of water at their disposal with which they can dilute substances such as ammonia.
However, birds and reptiles, at least lizards and snakes, are different.
It would appear that the conversion of their toxic nitrogenous waste products into a relatively insoluble one that can then be formed into a paste was an evolutionary adaptation. This enabled them to lead a terrestrial rather than aquatic life, and even to live in ecological niches where water is scarce.
In such niches it is particularly important not to have to find extra water with which to dilute toxic waste products and flush them from the system, so birds and lizards solved this by evolving to produce a paste of insoluble and relatively non-toxic uric acid.
Interestingly, birds that consume a lot of roughage with their diets, such as the heather-eating grouse and ptarmigan, produce droppings that are very similar to guinea-pig faeces. Only here and there among the droppings is it possible to make out the telltale white patches of their urine, so copious is their production of feces.
The evolution of insoluble excreta has nothing to do with a “good power-to-weight ratio” or the ability to “live in ecological niches where water is scarce”.
It evolved because all birds and many reptiles begin their life inside an egg.
Even heavy egg-laying amniotes that live in water as adults, such as penguins and crocodiles, must survive this early phase without poisoning their shelled enclosure with any water-soluble metabolites.
They do so from a great height because from a lower height it’s just too easy to hit the target, no challenge at all.
The deposit needs to be white so that, from said great height, they can see where it lands and who it hits.