Birds select nesting sites (remember, in the 'wild', they must make do with what is available) by several features:
1. Size of the entry hole - they don't want anything larger than they are to get in.
2. Space inside
Bluebirds require a 1 1/2 " entry hole and prefer houses that are stable/mounted. Smaller birds such as chickadees and wrens will use a smaller hole. We carry both mounted and hanging nest boxes with 1", 1 1/8" and 1 1/4" holes. We also have poles for mounting. Your birds will love you!
Bluebird Houses (eastern)