Ear mites, drives 'em mad. Got a little stray in that has scratched herself raw behind the ears and the tips of them as well. Had a quick look could not see anything, but she seems sore in her right ear, so treating for ear mites, if no improvement then off to the vets we go. No doubt, I will end up coming back with another stray!
Failing that have they been fighting? Ear tips, top of head and hocks and tail common places for cats claws to get caught.