"Steve Gilbertson of Minnesota has had good luck with bluebird boxes constructed from 4-inch PVC fitted with a wooden roof.  He is promoting these boxes as sparrow resistant.  Gilbertson feels sparrows avoid the slick plastic surface.  The boxes are mounted on a 1/2-inch conduit treated with carnauba car wax to predator-proof the pole.  I conducted tests on these boxes for several years and had moderately high bluebird use.  I have had almost no use of these boxes by house sparrows.  This is the box design I would most recommend for individuals having problems with house sparrows."