The black-capped chickadee uses snags of at least 4 inches dbh if making its own cavity. It rarely uses an existing cavity, but does accept nest boxes. The boreal chickadee may excavate several holes before choosing one for a nest. Occasionally, it uses natural cavities or old woodpecker holes. The red-breasted nuthatch usually excavates its own hole, in a tree at least 12 inches dbh, but sometimes uses an old woodpecker hole.
Many birds use old woodpecker holes or natural cavities exclusively.
These are the secondary users. The barred owl nests in a natural cavity in a snag with a minimum dbh of 20 inches and requires at least 250 acres of mature forest as its habitat, but prefers an even larger area. The Eastern screech owl needs trees with a minimum dbh of 12 inches. It uses natural cavities or abandoned woodpecker holes, especially those of the flicker and pileated woodpecker. It uses cavities for nesting, roosting and caching food.