Mount the box on a pole so that the entrance hole is eye level. All boxes should be of the type that will open either from the front or the side, both for monitoring and maintenance purposes. If you are putting up more than one box, the rule of thumb is placement of about 100 yards or more apart. This will ensure enough territory for a bluebird pair to find food for their young . This will ensure that parasites are removed from the box in the case of future nestings.
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I've used some of these innovative PVC boxes on my own trails and House Sparrows have avoided them. 39755 Highway 92, Carson, Iowa 51525-4275. Boxes should not be mounted on trees or other posts that will not allow the attachment of some type of predator baffle. Ensure that snakes, squirrels and raccoons are kept at bay by the use of a baffle. Flying predators are another problem.