I'm enjoying this - the fake scream spell was an especially clever idea.  Point of order, though - mockingbirds don't exist in Britain.  If you want a common bird that might be heard singing in London, go for a blackbird or a thrush.  Ravens don't come into cities here either.  That would explain why Snape assumed it was an Animagus, but if you want a corvid that might be there naturally, go for a rook.  That's a sort of large crow with a long beak, a pale patch around the base of the beak and loose feathers.  They always remind me of Snape - big nose, pale face and they look as if they're stalking along wearing a loose cloak.