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.
Build a house for a specific bird. Build a birdhouse for a cavity_nesting bird that occurs in your region and lives in the habitat in which you plan to put in a birdhouse. (See Birdhouse Dimension Table to select a species.) Build the right size house and hole dimensions. Different species of birds require different sized houses and entrance holes. Inside dimensions must be large enough to accommodate the incubating biard and a brood of growing young. Entrance hole size is especially important. If the hole is too small, the bird you built the box for won't be able to get inside. If the hole is too big, predatory birds and mammals will be able to get inside and reach the nest, and cavity_nesting birds may not use the house.