Actually, bird poop, per se, is not white. Birds, unlike mammals, do not have separate exits for urine and feces. Both waste products are eliminated simultaneously through the cloaca. While mammals excrete nitrogenous wastes mostly in the form of urea, birds convert it to uric acid or guanine, which reduces water loss in comparison. Uric acid thus forms a white sticky paste. So the white part is actually bird pee; it is the dark center that is the poop.
Since birds are supposed to have descended from dinosaurs, you may wonder whether dinosaurs peed and pooped the same way. It turns out that not all birds pee and poop simultaneously. Ostriches, for example, though they also expel waste through the cloaca, disgorge liquid urine before defecating. While it is not known for certain how dinosaurs excreted waste, some evidence has been found that suggests the possibility that non-avian dinosaurs also urinated and defecated sequentially.