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)

Wren Houses

Shop nesting boxes now https://order.wbu.com/buckingham/bird-baths-&-houses/bird-houses-&-nesting

Check out this video on Nesting Boxes-http://buckingham.wbu.com/content_section/show/501191