Use 2-cm (3/4") softwood lumber, such as cedar or pine. Do not use pressure-treated wood. It can be toxic to young bats and birds. Cut the front, sides, floor, roof, back, and partitions (see diagram). Bore an entrance in the front panel (4 cm in diameter for Bluebirds and Tree Swallows - 5 cm for Great-crested Flycatchers) 4 cm from the top. Drill small drainage holes on the side panels, just above the floor. Saw shallow, horizontal grooves (2 mm deep and 1 cm apart) on the inner surfaces of the bat section, including walls and roost partitions, to enable bats to crawl inside.