Birdhouse openings are quite often chewed on by squirrels. While they will not fit into the nest box, they do not know that until they have ruined your box. Woodpeckers can widen an opening by chiseling away at it. A metal portal protector can restore an opening to its proper size if the damage isn't too bad but adding one to a new box is an once of prevention. They come in three basic sizes: 1", 1 & 1/4" and 1 & 1/2"