Forest Preserve District of Cook County, Illinois
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Life in a Tree Hole
Nature Bulletin No. 581   November 21, 1959
Forest Preserve District of Cook County
Daniel Ryan, President
Roberts Mann, Conservation Editor
David H Thompson, Senior Naturalist

A forest is much more than just trees. It includes all of the underbrush, wildflowers and other vegetation that grow beneath these trees; as well as all of its animal life, both large and small. Sunshine, rain, wind, soil, and the leaf litter on the ground are part of it, too. A forest is a community -- a fabric in which the lives of its inhabitants are woven together and into their surroundings by a complex web of interrelations. Tree holes -- together with the birds, mammals and small life which they shelter -- furnish an important binding force in this forest community.

A hole in a living tree is usually caused by an injury which breaks through the bark and exposes the sapwood. This is followed by weathering and the infection of the raw wood by fungi and bacteria. Sometimes, before the wound is closed by the healing overgrowth of bark, the softened wood is invaded by carpenter ants and boring beetles. Many tree holes are started by ground fires which scorch the bark, by wind-broken branches, by lightning, or by cuts and bruises. Probably, most of them begin by the process of natural pruning in which the lower branches of a tree become starved for sunlight, die and break off. Most heal over but some develop cavities.

The woodlands in our Cook County forest preserves furnish many illustrations. If trees with holes were removed, we would lose most kinds of our woodland mammals and a large number of our common birds. Let us enumerate some local examples.

Four years ago last spring, a gray fox reared her three kits in the high deep hollow of an oak snag. In May, young raccoons are often seen taking a sun bath just outside the entrance of their mother's tree den. Opossums regularly sleep away their days in hollow trees, but sometimes, a female is found prowling in daylight with her mouse-sized brood of young clinging to her back. Other daytime sleepers in our tree holes are flying squirrels, white-footed mice, and three or four kinds of bats Fox squirrels and gray squirrels sleep at night and rear their young either in tree holes or in leaf nests called drays Some trees have both a day shift and a night shift.

The wood duck nests here more than any other duck. The eggs are laid, incubated and hatched in a tree hole. Then, as the hen calls, the ducklings tumble to the ground and she leads them to water. The sparrow hawk, screech owl, barred owl, and long-eared owl breed in tree holes; also, sometimes, the great horned owl. The nest cavity of a screech owl may contain hundreds of mouse skulls.

The resident woodpeckers of the Chicago region -- downy, hairy, red- bellied, red-headed, and flickers -- all nest in tree holes. So do such year-round birds as the chickadee, titmouse, and nuthatch -- as well as those imported pests the English sparrow and starling. Five summer visitors regularly come here to nest in holes -- the beloved bluebird and house wren, which seem to prefer the company of man; the tree swallow and prothonotary warbler (usually in dead trees over water); and the crested flycatcher, with the curious habit of draping a piece of snake skin at the entrance to its nest.

The little tree frog with its bird-like voice likes the cool interior of trees. So do swarms of wild honeybees. For years a group of scientists at Northwestern University has been studying and taking censuses of the hundreds of species of insects, mites, and other small life found in the mold inside tree holes.

All bird houses are merely imitation tree holes.

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