We had a few really mucky birds not that long ago, but, especially one in particular, they had been picking underneath the cars and one of them looked as if it had stuck it's head up the exhaust pipe it was black, It went about like that for the best part of a week, but I left it and after it having baths with just a little of milton sterelising fluid it's stains dissapeared and it's back to being white again. ;D