All birds set up territories around their nest sites and defend them against other birds. When House Sparrows or European Starlings lay first claim to martin housing at unestablished sites, they fill the compartments with their nests, then chase off investigating martins. Allowing House Sparrows and Starlings to nest in martin housing will significantly reduce martin occupancy and productivity. Starling-resistant entrance holes can be used to keep starlings from claiming martin housing. Housing should be closed up over winter to keep paper wasps, squirrels, and other birds from claiming the house before the martins return.
White housing seems to attract martins best.
White housing reflects the heat of the sun, keeping nestlings cooler. If your housing is unsuccessful, check the dimensions and modify where needed. Look for housing designed to raise and lower vertically, with easy access to compartments. If you are going to hang gourds under your housing you should stiffen up the bottom of the house some how. Landlords may need to lower housing daily to evict nest-site competitors, or to check on martin nestlings.