I just needed to post this
I put up a downy woodpecker house a few years ago. It's up about 10 feet off the ground and is about 12 inches deep. I live in a suburban area with not a lot of open space, so I was very surprised when a pair of eastern bluebirds moved in and raised a single chick. They never came back but i have had several birds check out the box. Never had the Downy's even looked at it as a potential home.

This points to one of my pet peeves; Bluebird houses do NOT need to be certain dimensions, pointed the right direction and/or only so far off the ground "or the birds will never nest in your yard"!! Bluebirds are cavity nesters normally as are woodpeckers and several other birds. They will nest in holes in trees 30 feet or more off the ground if that's what's available! I have a friend with a decorative jar on the railing of his from porch (it lays on it's side) and he has bluebirds that have reared up to 4 broods a year in it for the last 3 years or so. That's not to say you shouldn't put up bluebird houses, but don't worry that you don't have everything just right, the birds will nest if they like it, regardless of whether it's 'right'.

ON a side note, watch the bluebirds around the nestbox. They will not attach Titmice, Chickadees or a host of other birds that might encroach even right on the box, but let a squirrel or woodpecker get with feet and watch the fur fly! They won't share.