Larger compartments are also attractive to European Starlings, but a special entrance hole will minimize starling problems. Make sure there is adequate ventilation and drainage in each compartment. Many plans for martin housing, and some manufactured houses, are made to incorrect dimensions, so if your housing is unsuccessful, check the dimensions and modify where necessary. Most houses can be improved. Add insulation to the attic, remodel interiors to offer double-size compartments, and add porch dividers.
Dividers help keep males from claiming extra compartments, and can double occupancy rates.
They also keep nestlings from wandering to other compartments, where they can get lost and die, or steal food from younger nestlings, causing them to starve. Protect your housing from predators. Don't assume that because you never see any predators there are none around. Raccoons, snakes, and owls raid bird houses at night. Few experiences are more painful than losing all your martins because you didn't equip your poles and housing with guards.