Bluebirds most frequently nest in dead or aging trees. https://www.flickr.com/photos/chuckthephotographer/4534699765

Bluebirds have several challenges

Lack of natural habitat
Many housing developments remove these old trees leaving the bluebirds with few options. It would be best to recover the natural housing opportunities over time, through development practices that contribute to habitat improvements.
Invasive species competition
Some invasive species (ex: house sparrows) have been outcompeting bluebirds for nest sites.

This nest box provides a good option in light of these challenges.

The opening is small enough to discourage nesting by invasive species such as house sparrows.