Winter is coming
In the winter birds will seek shelter from the elements. While most birds will find refuge in and evergreen, cedar or fir tree your cavity nesting birds such as Bluebirds, Woodpeckers and Wrens may use a nest box. This habit is known as roosting. They won't build a nest inside but just huddle within. Sometimes there will several birds of the same species cuddling up for extra warmth. Make sure all the old nests from summer are cleaned out. You may see them outside or near the box at dawn. I have a Downy Woodpecker who comes out of the woods each night to occupy a Bluebird box.