Purple Martins are highly social, cavity-nesting birds. In the Eastern United States, a shortage of natural housing options has left these fascinating and attractive birds completely dependent on man-made housing. Author Holly K. Ober tested seven commercially available martin houses to determine what is best for martins in north Florida and provides recommendations in this 4-page fact sheet published by the UF/IFAS Department of Wildlife Ecology and Conservation.
http://edis.ifas.ufl.edu/uw450