Bird Crap Mark! :(

[Niall]

It's the amonia in the bird shit because bird shit isn't just 'shit' it's bird piss also, that's what eats into the paint work, the extra heat just accelerates the process

Exactly this. A good layer of wax will help protect to an extent but doesn't make it immune!! AG do a pack of wipes you can bung in the glove box to wipe it off.

If you want to look a tit and go and buy a Matt black M3 or similar, you have to be so stupidly careful with the paint. Get bird shit on it for a few days an it literally ruins it.
Clean it and leave water marks on the surface and they will ruin the finish!