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