Not sure if it will help, but we have a similar issue here at BMW, Where we store our cars seems to be a magnet for bird shit.

If it is in an isolated area, and not all over the car, what our head valeter does, is get a heat gun, and from a distance, runs the gun over the affected spot a few time, lets it cool, polishes it, the repeats, the heats helps soften the paint, and then the polish will help flatten it.