Is P21 a type of Meguiar's polish?johnhab said:Birdsh!t is highly acidic and contains all sorts of abrasive stuff they eat, like seeds, gravel, sand, etc.
Anyway, the acidity etches the paint, so you actually have to polish the spot out. Buy the mildest polish you can (Meguiars, P21 if they make it, Griot's, etc.) and you should be able to remove it no problem. Of course rewax it after you're done.