Basically all animals have nitrogenous waste that they need to get rid of. How they get rid of it varies based on their environment. Fish just release it as ammonia, because they're constantly surrounded by water, so they don't need to put any more effort into it. Mammals concentrate their nitrogenous waste into urea, which packs more nitrogen into each molecule, so we can store more in less water. Urea is a fairly good packaging system for us, because it allows us to put our body's waste in a modest volume of water, and carrying around a modest volume of extra liquid doesn't really interfere with our ability to move around and do stuff. Birds fly, though. For them, weight is mission critical, and they've evolved to turn their nitrogenous waste into uric acid, which packs even more nitrogen into a molecule than urea. Also, uric acid is white.
TLDR: birds store waste differently to save weight, the stuff they turn it into is white
Bird's excrement is actually a combination of their "urine" and feces.
Birds don't "pee" like mammals, they excrete all waste from the same orifice called a "cloaca," which is like a fancy butthole.
The white junk is nitrogen and urea and other stuff that would normally be excreted in urine in other animals.
We mammals excrete nitrogenous body waste (somewhat toxic) in urine because we can store a lot of water fairly conveniently and this dilution makes the waste less internally harmful. Birds can't carry much water or they would be too heavy to fly, so their kidneys have adapted to produce more concentrated dry nitrogenous wastes requiring less water waste.
Source: b.s. in biology
While everyone else is correct that bird waste includes both feces and urine, they are also forgetting something else super important: rocks.
Birds don't have teeth. They break down their food with a gizzard instead. When a bird eats, they eat teeny-tiny rocks, too. Instead of going into the stomach, the rocks go into the gizzard. The gizzard is wrapped around a special part of the esophagus, and the rocks are rubbed and grinded on the food as it makes its way to the stomach. When a rock in the gizzard has been pulverized into dust, it passes into the waste of the bird as a rock-water sludge.
And that pulverized stone is white.
Because it's their urine as well as their poop. The concentrated urine content of birds is a white paste. (If it were highly watered down like mammals it would also be a light coloured fluid)
their urea- pee is white.
their poo is brown.
same with snakes and reptiles. They only have one hole, so it's a poopee
Uric acid is a combination of feces and nitrogenous waste that streamline the disposal of unwholesome chemical elements.
It's not actually feces- the white part is actually basically their urine. The darker matter is their poop
Bird feces are actually a mix of feces (brown) which is the remnant of undigested food and uric acid (white and crystalline) which is a product of protein metabolism (nitrogenous waste). Uric acid is a concentrated form and does not require to be dissolved into lots of water for safe excretion (humans excrete urea which needs water and fishes excrete ammonia which requires much larger quantities of water).
So I don't do bird medicine (or bird law) but human feces is colored the way it is partly due to the presence of bilirubin, which is unsurprisingly a dark yellow-brown sort of color. Presumably avians manufacture a different sort of bilirubin, or less is in their excrement, or etc etc. But the reason our poop is brown is because of this product produced by the liver.
Fun fact, during huckleberry season in Montana, the rocks are covered in purple/blue bird poop.
The white is actually dried up urine. The black is feces. If you look at fresh bird poo (I know) you will see there is no white.