Tree cavities are formed by tree disease and decay and by woodpeckers. All six woodpecker species in Alaska excavate cavities for nesting and roosting. Most other cavity_nesting birds use abandoned woodpecker holes. Usually a pair of woodpeckers excavates a new hole every year. Before selecting a final nest site, a pair may start, then abandon, several holes. During fall, overwintering woodpeckers also excavate cavities for winter roosting. However, woodpeckers are only able to excavate holes in dead or decaying trees.
Birdhouses should be placed in fall or winter, if possible. Swallows may use nest boxes set out during late May or early June, but owls, chickadees, and woodpeckers begin looking for nest sites during February, March and April. Always face the birdhouse so the entrance is protected from prevailing winds. Kestrel nest boxes should be placed at the edge of a forest area or on a post in an open area 12┰ feet above the ground.