Why are birds the only animals that have white poop

They eat similar organisms as do other species. What's the difference?

the white substance in the feces of birds is called uric acid. Animals that excrete uric acid are known as uricotelic. Excreting uric acid is one way to conserve water, because the alternative of excreting urea or ammonia require more water. Insects, and reptiles such as lizards, snakes and crocodilians also excrete uric acid, whereas turtles, mammals, amphibians and sharks mostly excrete urea, and fish excrete mostly ammonia. Uric acid does have its disadvantages because it takes more energy to synthesize but it is less toxic than urea or ammonia. Ammonia is highly toxic and requires a lot of water to get rid of, which is not a problem for most fish since they live in water.

The ability to excrete urea is probably inherited by the primitive reptiles from their amphibian ancestors. The ability to excrete uric acid among some reptiles probably originated in the amniotic egg, or shelled egg. Since uric acid is not soluble in water, it can be safely stored in the shelled egg while an embryo is developing. Adult mammals also synthesize a small amount of uric acid. If too much uric acid accumulates in our blood, however, it can lead to gout, when the uric acid crystalizes and are deposited in the joints.

Birds excrete uric acid because they are descendants of a diapsid reptile. Other living diapsid reptiles, such as snakes, lizards, and crocodilians also excrete uric acid. Therefore birds likely inherited this trait from their reptilian ancestor.

They aren't. Hyenas also do from the large amount of calcium in their faeces.

not 100% sure but they pee and poop at same time..maybe that has something to do with it...