Many species use them for nesting and roosting

The creation of tree cavities begins with the red-cockaded woodpecker. This is the only woodpecker in North America that excavates its roosting and nesting cavities in live mature pine trees. It selects the largest pine tree within a tree stand, ranging from 30 to 60 years old. The creation of a cavity in a live pine tree takes from one to three years. Cavity excavation is more successful in a pine tree infected with red-ring rot, which is caused by a fungus. The red-cockaded woodpecker may eventually abandon the cavity it created to relocate somewhere with better ecological conditions, or it may be forced out of its cavity by another species.

Cavities in the bird world are exceedingly rare, and competition is fierce for this limited resource. Some competing species are the red-bellied woodpecker, northern flicker, pileated woodpecker, red-headed woodpecker, eastern bluebird, bat and southern flying squirrel. Other birds using cavities include the chickadee, nuthatch, vulture, owl, merganser, some large ducks, the flycatcher and many other species of woodpeckers. 

 In all, 85 bird species nest in tree cavities in North America. Cavity nesting is safer and more successful than nesting on the ground or in a twig nest placed in a tree or shrub. The small entrance to a cavity is much less noticeable and easier to defend from predators such as the rat snake. An incubating or brooding woodpecker can easily defend its eggs or young against a tree-climbing rat snake because of the small cavity entrance and the woodpecker’s powerful neck muscles and sharp stiletto bill. 

A tree cavity is used year-round for roosting and shelter from weather and predators, as well as nesting. Like a house, a cavity provides excellent shelter against weather. Wood provides great insulation against hot and cold temperatures. The thick wood and shade of a large tree or snag (a standing dead tree) moderates the heat from direct sunlight. The interior of the cavity also remains dry during rain unless wind drives the rain into the cavity. (During my time in the field as a professional ecologist and nature photographer over the past 50 years, I have observed woodpeckers and eastern bluebirds blocking the entrances of their cavities during rainstorms, thus preventing the cavities from getting wet.)

Except for the red-cockaded woodpecker, most woodpeckers excavate their nest cavities in dead limbs of living trees or in dead trees. Dead wood is softer and easier to excavate, but it must be firm enough to provide shelter from weather and protection from predators. Squirrels and raccoons can destroy a cavity if the wood is too soft. 

The importance of standing snags for nesting and roosting to wildlife, especially birds, cannot be overemphasized. They provide food and shelter to invertebrates, amphibians, reptiles, birds and mammals. Many birds depend on injury, decay and disease for trees to form a natural cavity in a snag. When the interior wood of a snag becomes soft enough, most birds can enlarge the interior of a cavity and its entrance to make a more suitable chamber. Nesting cavities are usually excavated by the male. 

As a cavity increases in size over time from excavation, there is a continuum from smaller inhabitants to larger species. For example, a cavity may first be used by small chickadees followed consecutively by larger species such as nuthatches, downy woodpeckers, red-bellied woodpeckers, pileated woodpeckers, eastern screech owls, barred owls and so on. 

A nesting cavity cannot be excavated too far in advance of the nesting season, as larger species could find the cavity unoccupied and increase the size of the entrance and interior. The cavity would then become useless to the originator. The cavity originator can also be usurped from its nest cavity by a smaller but aggressive species such as the European starling, which will occupy the vacant cavity. Therefore, a cavity is usually constructed within a week or two prior to laying the eggs. 

Cavity-dwelling birds are important, as they help control destructive and harmful insects and rodents and prevent the use of expensive and harmful pesticides. For these reasons a standing snag should never be cut down unless it poses a danger to property or for safety reasons.

William R. Cox has been a professional nature photographer and ecologist for more than 35 years. Visit him online at williamrcoxphotography.com.