Looks like there really is no way around this. This is a quote from a detailing guru:-
"Bird lime is incredibly corrosive. It'll eat through wax, sealant and paint given the opportunity. The key is to remove it the instant you find it. If its etched the paint then it will need compounding to remove the damage.
The other mistake that most people make is rubbing it to get it off. Bird lime will contain tiny piece of grit, so when you rub it against the paint it'll mark like hell. I always flood the area with glass cleaner or quick Detailer, allow it to soften for a minute or two and then very gently wipe it away with a soft cloth. If it doesn't come off cleanly, soak it again. It's worth being patient with."