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

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

  • This good-natured column is written by Pam Otto, manager of nature programs and interpretive services at the Hickory Knolls Discovery Center, a facility of the St. Charles Park District. She can be reached at 630-513-4346 or

“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. A little topical anesthetic, the pressure of a needle and the numbing effect of novocaine all combine to make for a procedure that is not altogether unpleasant.

(Now if only science could find a cure for dentists who ask us questions as we’re undergoing a procedure … )

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.

These dark hollows, created when branches drop or excavators like 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. 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 too will 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 record number — 88 — were recorded a few years ago during Kane County Audubon’s Christmas Bird Count, and since then several people have told me about putting mealworms out in winter for the bluebirds that come to their birdfeeders.

Perhaps the bluebirds have found, as did their cousins the robins, that winter food sources are fairly abundant in Kane County. 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. The bur oaks behind Hickory Knolls provide endless entertainment for our visitors, as each has at least one hole and all are currently in use. Red-bellied woodpeckers occupy one and honeybees are in another.

But none are in as high a demand as the one that overlooks our birdfeeding station. Situated 20 feet off the ground and 30 feet from the feeders, this hollow is 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 like raucous raccoons from forcing their way in.

Useful as they are to wildlife, tree cavities also provide a valuable service to humans. 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.

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.

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