Hi there Greengardener. There is a lot of responsibility in hosting cavity nesting birds. First thing you want to do is see what kind of birds you have. Then you need to pick the right box for the species. You do NOT want to put the box on the tree. I am sending you a website that says it is about bluebirds, but it is really for all cavity nesters. Chickadees, Titmice, Nuthatches, Tree Swallows...just to name a few. Spend a little time to see what is involved. So many people (myself included at the beginning) think you can just slap any cute box up on a tree and have successful nesting....In truth they are better off nesting in a natural cavity if the box, placement, and surroundings are not right. www.sialis.org. Hope this helps!

In short, you can mount them on a post, preferably in the open where predators cannot get to them (6' or higher) and I know bluebirds prefer the opening set in an easterly orientation.
-Bonnie

If you don't mind the reading http://sialis.org/ is a great site.

3/4" electrical conduit (10' length cut in half) over a length of rebar pounded about a foot into the ground. Mount the box to the pipe with a 3/4" bracket. This helps with the predator problem. Some people steel wool the pipe and coat with car wax. If you attach to trees, you get predators-snakes, squirrels, cats-to name a few.

You need to check the boxes periodically to make sure you are not hosting a non native hostile species (like the house sparrow). Generally, if you can't monitor the box, you are doing a GREAT disservice to our native songbirds, blue birds included. Please do the research-it is well worth it. You can avoid a lot of heartbreak with a little education. The blue bird forum is also a great site full of very helpful folks,