Prevention is best
Prevention is the best cure for this one. Firstly, before you park your car, check the ground; if you see signs of birds resting above (bird poo on the ground) - park elsewhere. Sometimes though, it's not from parking under a tree, but a dive-bomb attack in the middle of nowhere. So for these, it really pays to be prepared by having a good barrier on your paint. Make sure your car is waxed often as this will slow the acid moving into the paint, but as we have found, hardcore poop (like that from bats) can go through waxes, sealants, and even nano based coatings with ease. So an extra layer will just give you more time, it's still the most critical thing to remove it as fast as you can from your cars' paint.
This is why we recommend people have a microfibre cloth and bottle of the Clean Detail in their cars at all times. If you can get to it quickly enough, you will be in luck and there should be no damage to the paints' surface. If you have access to water or a hose, get that on it ASAP, but for many, it happens when we are far away from home so the Clean Detail becomes your best bet.