Bird droppings combine the whitish waste products processed by the liver and kidneys with the darker wastes that come from the digestive tract.
In birds, the nitrogen-rich wastes are turned into a white-colored paste that is composed mostly of urates. The urates often form a separate white blob, as anyone who has worn a new hat outdoors at the wrong time has probably noticed.
The excretory system of birds is different from that of mammals because it takes up a minimum of space and requires a minimum of water for processing.
In mammals, nitrogen waste is turned into urea, a toxic substance that requires large amounts of water to flush it safely from the system and a urinary bladder as a holding tank for all the liquid.
The urates excreted by birds, on the other hand, are concentrated but not toxic, and most birds do not have bladders to hold liquid to flush it out.