When you find a bird dropping on your car ( best to find 'em and try to eliminate 'em as soon as possible ), get some plain water ( mineral water pun boleh wan ) and plenty of tissues. Put plenty of tissues on top of the bird poo, then thoroughly wet 'em. Leave it for about at least 15 - 20 mins ( depends on how bad the bird poo is ), then, grab more tissues, and use 'em as 'gloves' to pick up the now almost drying tissues + bid poo, while pouring more water while doing so. This will help to clear off the bird poo and in the same time, reduce the possibilites of scratching ( it'll still scratch la, but not much ). Best practice is to wax your car as often as once a week. The wax will become a sacrificial protective barrier on the car's paint.