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

****:LIFE IN A TREE HOLE

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.