Prevention is better than cure. If you already have damage, you need to get it fixed.
If you park on the road, avoiding droppings will be impossible. In this case, get a car cover for parking for extended periods of time. What I do when I dont take the car out for some reason is I inspect the car once I get home. I have a box of tissues in the car, so if I have droppings, I pour water then just wipe off after a few minutes. Also, having teflon overcoat done helps the clean up
Ameyam