Bird poop marks are caused largely because bird poop is white and sets hard as it dries. So on a hot day the white poop lands on your preheated soft lacquer and sinks into it, then sets hard. So when you clear it off what is left is simply a depression in lacquer. Therefore gently heating the lacquer again lets in flow back to level, done correctly the mark literally melts away.
How hot do you need to get it because here, on a hot day with a black car, the exterior temperature can easily fry an egg, or cause you serious burns yet I still see cars with damaged paintwork caused by birds?