House sparrow nests should be removed from bluebird boxes immediately. House wrens, chickadees, nuthatches, tufted titmice, and tree swallows may also use the boxes. If these species begin to occupy too many of the boxes you can add more houses to the trail or double them at each site. To prevent predators like raccoons, opossums, and house cats from climbing the nest box poles, put tin around the pole at the base of the box. Grease can be placed on metal poles to prevent predators and ants from climbing. Entry holes can also be extended with wire mesh or wooden collums to reduce predation.