This is a bat house. Bats often roost in hollow trees, in the eves of houses, and of course, in belfries. A bat house gives the the perfect place to roost. The screen on the lower part gives them a secure place to land and take off. The house is wide enough for the bats, but too narrow for most predators to get in. There are quite a few places to get plans. I found a good set on the Carlsbad Caverns site at: http://web.archive.org/web/20050407073827/http://www.nps.gov/cave/bathou...