4. The Flap
Some nooks that don't become sticky, develop simple flaps that cover the entrance. This serves to disorient the mouse and make his capture easier. Because he is eating the cheese, it allows us more time to approach the entrance (we no longer have to wait there). Since the flap blocks the mouses line of vision, it can not see us coming and does not run away.
Note: This mutation is successful only after the bait already exists. Flapped nooks without bait do not attract many mice or keep them preoccupied long enough for us to sneak up on them with our shovel.