Demelza smiled at the girl's concern, which she was quite familiar with. The Owlery was definitely not one of the cleaner places in the school.
"I know what you mean," she said as she glanced at the back of the Ravenclaw's head and down the back of her sweater, "I think Filch avoids cleaning it for exactly that reason." She pointed her wand at the one spot on Lisa's back and muttered, "Scourgify." She then spoke more clearly, "there, you're good to go."