Thud! My walking stick hit the base of a bleached-out elm along the edge of a river slough. A furry, gray form shot out of a hole and scampered up the remaining trunk. Like a nervous high diver, it paused momentarily on a branch near the top before leaping. With front and hind legs outstretched, it glided down at an angle and landed on the trunk of a nearby cottonwood.
After tapping on dozens of hollow trees, I'd finally rousted my quarry, the southern flying squirrel. These endearing little animals, with their silky gray brown fur, big black eyes and sail-like flap of skin between front and hind legs, are just one example of the many cavity creatures that live in hollow trees.
When we think of forests and woodlands, we commonly focus on living trees. But the death and decay of these trees is an integral part of the forest ecosystem. As trees grow older, they increasingly become susceptible to damage from disease, insects, strong winds, lightning and floods. Damaged areas on trees provide entry points for fungi, which colonize the heartwood and eventually consume it, producing hollow trees.
Hollow trees provide homes for wildlife. In Missouri they are used as nests or roosting sites by 26 bird species. Most of these species use existing tree cavities, but woodpeckers actually excavate their own. These predominantly black and white birds, often with bright red markings on their heads, use their strong claws to grip tree trunks. With their stout bills, they hammer away at decaying wood and hollow out nesting chambers inside trunks and limbs.
Each woodpecker species constructs a slightly different style of nesting cavity in terms of the size of the cavity, diameter of the entrance and height above the ground. Downy woodpeckers always select dead wood and create a circular hole about an inch in diameter. The somewhat larger hairy woodpeckers, on the other hand, prefer live wood and make oblong holes about 2.5 inches high and 1.5 inches wide.
Northern flickers, still larger, construct a 3-inch hole that is 8 to 25 feet above the ground. Missouri's largest woodpeckers, the pileated, chip out rectangular or oval entrances up to 5 inches high and from 15 to 70 feet up the trunk.
Because woodpeckers construct their own cavities, they are called primary cavity users. Animals that don't excavate their own cavities, but use existing holes, are called secondary cavity users. Many of these non-excavating species will use or enlarge natural passages into the interiors of hollow trees, taking advantage of fire scars, broken limbs or other injuries. Secondary users also will use holes and cavities constructed by woodpeckers.
Natural cavities for bluebirds are in short supply. Before settlement, bluebirds nested in hollow snags in prairie groves and in the open savannas that covered much of Missouri. With the loss of savanna habitat and with the new dominance of homesteads and small farms, these birds began nesting in dead trees and rotted fence poles.
But as farms and fields grew larger in the latter part of this century, landowners removed the wooden fence poles. New fences made of steel or treated wooden posts lack cavities, and bluebird numbers declined. Conservationists designed nesting boxes and placed them along field borders and fence rows. Bluebirds started using them immediately.
Chimney swifts and purple martins rely on artificial cavities. Chimney swifts once nested in large hollow trees, but now they nest mainly in chimneys, wells, silos and similar structures. Martins also nested in hollow trees but now generally use martin houses put up by homeowners.
Many cavity nesters need hollow trees in specific sites. Tree swallows and prothonotary warblers generally use cavities located over water. Several ducks, including wood ducks, goldeneyes, mergansers and buffleheads, use hollow trees in or near water.
Barred owls usually use cavities located in bottomland forests, while great crested flycatchers prefer cavities near clearings or woodland edges. Great horned owls, screech owls, chickadees, nuthatches, wrens and titmice use cavities in a variety of forest and woodland habitats.
Birds use cavities to protect their eggs and young from predators. Red cockaded woodpeckers, once abundant in the Missouri Ozarks, would excavate cavities in live old pines and then create a series of smaller holes around the entrance. These smaller holes oozed pine resin, forming a sticky barrier that made it difficult for snakes to reach their nests.
Some species select a cavity with the smallest opening they can squeeze through. A small opening excludes nest predators, such as blue jays and raccoons and nest parasites, like brown-headed cowbirds. It also bars larger cavity users that would otherwise compete for a particular cavity. The great crested flycatcher often hangs a snake skin in the cavity entrance. The skin works like a scarecrow, frightening away other cavity nesters.
Squirrels, opossums and raccoons den in hollow trees. Other mammals, including bats, striped skunks, mink, gray foxes and black bears also will roost or den in tree cavities. Hollow trees provide shelter from the elements, as well as sites for raising young.
Raccoons prefer an opening 4 to 10 inches wide that is located 10 to 40 feet above ground. Gray foxes occasionally den in hollow trees and even climb them. They are known to climb vertical tree trunks to heights of 60 feet. They back down a vertical trunk or turn around and run down a sloping one.
Black bears often den under tree roots, debris piles and hollow logs, but sometimes prefer elevated dens in hollow trees because they offer better protection from cold winter rains. How large would a hollow tree need to be to house a black bear? Adult males can weigh as much as 600 pounds.
The southern flying squirrel, a secondary cavity user, is selective in its choice of cavities. While they will use a variety of cavities that are 20 to 30 feet above the ground, they prefer those constructed by downy woodpeckers. They usually line the cavity with shredded bark, but also will use lichens, moss, feathers or leaves.
Cavity users aren't limited to birds and mammals, or even vertebrates. Honeybees use cavities, and so do carpenter ants. These non-stinging ants are light red to brown or black and from 1/4 to 1/2 inch long. They build their nests in the heartwood of dead trees, logs and stumps by chewing through the interior of the tree and creating an intricate network of tunnels. Each colony contains about 2,500 workers, some males and a queen.
The Conservation Department recommends leaving or establishing a minimum of seven hollow trees per forest acre. These trees should be marked and saved when harvesting timber. If no suitable trees exist, girdle or wound selected individuals to create snags or hollows in future years.
The Conservation Department also manages some public forests, woodlands and savannas by mimicking the natural disturbances, such as fire and flooding, that have maintained Missouri's rich array of species over thousands of years. This allows the ecosystems to create and maintain snags and hollow trees to sustain cavity nesters, along with the rest of our native plants and wildlife.