Illinois Department of Natural Resources
November 2021
November 1, 2021
Photo by Elisa Stone.

Good Natured: Tree Cavities Provide Life-Saving Protection During Harsh Winter

By Pam Otto

“You have a cavity.”

For many of us, delivery of this statement evokes feelings of disappointment and maybe even gloom. Our dental hygiene regimen—all that brushing, flossing and rinsing—has somehow failed, and now there’s a hole that doesn’t belong. The good news is that modern dentistry makes treatment of cavities virtually painless.

But enough about “bad” cavities. Today we’d like to spread the word about cavities that actually are good. Phenomenal, in fact. For they provide shelter and save lives.

We’re talking, of course, about tree cavities.

A woodpecker with a red head and black body perches on a cavity on the side of a tree trunk. In the background is another tree trunk.
Photo by Michelle Smith, USFWS.

These dark hollows, created when branches drop or excavators such as woodpeckers go to work, provide near-total protection from the elements for many local mammals and birds.

Sleety rain? No big deal when you’re inside a tree cavity. Likewise for wind-driven snow. Even below-zero temperatures are moderated by the heat of warm bodies piled together and surrounded by layers of wood and bark.

Raccoons, opossums, flying squirrels, chickadees, nuthatches, woodpeckers and screech owls are just a few of our year-round species that take shelter inside tree cavities.

Spiders and insects also take advantage of the accommodations within a weathered tree nook.

Now, I haven’t tried this in winter, but in summer one of my favorite night-time activities is to aim the beam of a flashlight inside a dark tree cavity. More often than not I’m treated to tiny points of light glaring back. These bright dots are the eye shine of spiders who have sought haven inside the tree (and who probably curse the invention of the flashlight. And naturalists.)

Tree cavities are also proving useful for another summertime species that is increasingly common here in winter: Sialis sialis, or the eastern bluebird.

A gray screech owl closes its eyes and rests at the opening of a tree cavity in a dead tree.
Photo by Pete Nuij.

Perhaps the bluebirds have found, as did their cousin the robins, that winter food sources are fairly abundant in locations throughout the state. Goodness knows we have no shortage of crabapples and berries, especially those on nonnative honeysuckle shrubs.

Plentiful food is reason enough to forgo a taxing migration; add in a supply of sheltered roosts and the decision to stay here is a no-brainer.

Provided the tree itself is sound, cavities that face windows of nearby buildings are treasures indeed. Watching red-bellied woodpeckers and honeybees sheltering in bur oaks provides endless entertainment.

In one area, situated 20 feet off the ground and 30 feet from birdfeeders, I have found the Taj Mahal of tree cavities. It’s spacious enough to hold at least four chunky fox squirrels, shielding them from weather and predators alike. Several times a day we’ll look out to see the rotund rodents scampering out, one after another, like furry circus clowns streaming from a tall, vertical car.

What’s more, this cozy home has its own built-in security system. The entry point is only a few inches in diameter—large enough to admit the squirrels, yet small enough to prevent interlopers such as raucous raccoons from forcing their way in.

Useful as they are to wildlife, tree cavities also provide a valuable service to humans. It seems logical that the more hollow trees there are to provide homes for mammals and birds, the fewer animals there are living in attics, eaves, garages and sheds.

A squirrel pokes it head out of a tree cavity. In the background is green ivy.
Photo by George B.

Years ago, folks used to fill holes in trees with cement, and sometimes brick and mortar. Not only did this practice deprive animals of homes, it also hastened the tree’s destruction by interrupting natural processes.

And heaven help the arborist who would eventually have to take down that concrete-laden trunk.

These days, for the most part, people let the holes in their trees be. These are the sorts of cavities we want to encourage—no filling required.

Pam Otto is the outreach ambassador for the St. Charles Park District. She can be reached at (630) 513-4346 or

The images is a logo for St. Charles Park District. The logo has a person riding a bicycle on top of a leaf which is all over waves at the bottom of the graphic. Three flying birds are in the background at the top.

This article is adapted from Good Natured, a column that runs in the Kane County Chronicle. For more nature-related articles, visit the Good Natured page.