The droppings produced by birds contain materials from the excretory (uterine) and alimentary (faces) systems, a result of the two emptying into the cloaca, where the reproductive system also has an opening.
Whereas the faces contain undigested food remains, and so tend to reflect the diet in general composition, the urine is responsible for the white appearance seen in many bird droppings.
The urine produced by bird varies widely in appearance, but is usually cream-coloured. It contains a high concentration of urates – the salts or esters of uric acid – and whitish crystals of uric acid are deposited after some of the water has been reabsorbed in the cloaca. It is these crystals that give bird poo its white appearance.
The whitest droppings tend to be produced by seabirds and it is these, rather than those of other birds, that may be correctly termed guano. Some species such as cormorants, use guano as a nesting material
To help you discover which bird the poo belongs to check out our handy guide to identifying animal and bird droppings