Cavity-nesting eastern bluebirds use naturally occurring holes in trees, cavities hollowed out by woodpeckers, or man-made houses. You can maximize your chances of attracting bluebirds long-term by providing appropriate housing options and locating the houses in suitable sites. This 3-page fact sheet written by Holly K. Ober and published by the UF/IFAS Department of Wildlife Ecology and Conservation provides recommendations on housing for bluebirds in north Florida.
http://edis.ifas.ufl.edu/uw451