Only 86 species nest in natural cavities. One way a natural cavity can be formed is by a primary nester excavating one.
Woodpeckers are primary cavity nesters.
A primary cavity nester excavates a new hole each breeding season. The holes are then occupied by other species of birds. Species of birds that rely on pre-existing holes are called secondary cavity nesters. Secondary cavity nesters included bluebirds, wrens, swallows and flycatchers. Another way a natural cavity can be formed is by decay.