Why is bird poop mostly white?
The Audubon (Autobahn ?) site has the answer, but it’s way too early for sincere replies and this could/should get funny.
Welcome !
Bird poop is dark brown or black, just like most mammal poop.
Bird pee, however, is white.
As to why it’s white rather than yellow, and a much thicker consistency, it has to do with saving weight, though I don’t remember all the details.
ISTR that birds have one opening, called a ‘cloaca’ that all their waste comes out of, and that their pee is mostly uric acid, which is basically little white crystals. Apparently by converting their nitrogenous waste into uric acid, they can reabsorb more water and save weight or something vs. converting to urea like mammals do.
Right. Animals need to dispose of nitrogenous waste formed by the breakdown of proteins. The compound used by mammals is urea, which is rather soluble and disposed of as watery urine. Reptiles and birds instead use uric acid, which is a white crystalline compound that is not very soluble and disposed of as a whitish paste. Reptiles and birds are generally much better at conserving water than mammals. As mentioned, another advantage is that birds don’t have to carry around the weight of liquid urine before disposing of it, although that is not why it evolved, since reptiles also excrete uric acid.
Hummingbirds are an exception, since they eat a mainly liquid diet and thus excrete largely liquid urine. However, they still dispose of nitrogen in the form of uric acid.
In a bird dropping, you will often see a dark lump in the middle, which represents the feces, surrounded by the white urine.
When the wild blackcaps (berries) are in season, it’s often mostly purple.
Likewise for mulberries. Love them or hate them, the place you do NOT want to have a mulberry tree is right over your driveway.
I think you have it backwards. Board policy is that the sincere replies should substantially answer the question before joke replies are posted.
Ah. I didn’t know that. Thank you 
“My teacher once asked me what kind of animal I would want to be, if I could be anything. I said a bird. She said, ‘Why, so you could fly?’ And I said, no, so my shit would be white.” - Steven Wright
Now that our resident ornithologist has helpfully rendered the actual factual answer …
No discussion of bird droppings is complete without a reference to this classic that’s highly informative in its own way. Really!
https://what-if.xkcd.com/11/