The Secret Air Conditioning Inside Every Old Tree

A hollow in an old tree looks like a hole, but for a squirrel, an owl or a bat, it can be a shelter from heatwaves and cold snaps, and it does this with no wiring, no fuel and no maintenance. New research suggests just how well these hidden chambers work. It might change how we think about forests, wildlife and even our own architecture.

What You’ll Learn

• How tree cavities buffer the temperature swings of the weather outside
• How woodpeckers, bats, owls and squirrels rely on these hidden chambers to survive heatwaves and cold snaps
• What this means for forest conservation as the climate changes
• How tree cavities offer a biomimicry blueprint for better shelters, both natural and human-made

A Climate System in Plain Sight

A recent global review pulled together data from dozens of studies across five continents, all measuring one simple thing: how much warmer or cooler is it inside a tree cavity than in the air outside?

The pattern held almost everywhere. During the hottest part of the day, cavity interiors were cooler than the surrounding air, and the hotter it got outside, the bigger the gap became. On the coldest days, the same hollows stayed noticeably warmer than the outside air.

A tree hollow isn’t just a hiding place. It’s a passive thermostat, and it doesn’t hold a perfectly steady temperature. It simply softens the extremes, which is often the difference between coping and not coping.

That matters because heatwaves have already killed tens of thousands of animals in single events. Flying foxes dropping from their roosts in Australia are among the grimmest recent examples. As heatwaves become more frequent and intense, somewhere cooler to retreat to isn’t a luxury for wildlife. It’s survival.

Meet the Tenants

Woodpeckers: the original architects

Woodpeckers don’t just use tree cavities. They build them. Many species chisel out a fresh chamber each breeding season, and the old ones are later taken over by dozens of other animals, known as secondary cavity users. Woodpeckers also roost in these hollows themselves, so they benefit from the same shelter they create.

The host tree shapes the result. Woodpeckers change how they drill, the shape of the hole and the timing of their nesting depending on whether they’re working in a cactus, a pine or a broadleaf tree. Take the Gila woodpecker, which excavates nest chambers in giant saguaro cacti in the desert. The cactus’s thick, moist flesh makes a very different wall from dense hardwood, so the temperature inside behaves differently too.

Every hollow a woodpecker carves is, in effect, climate infrastructure for a whole forest community. You can read more about woodpeckers in Amazing Creatures to Spot in Autumn and Woodpeckers: Fantastic Long Tongue and Amazing Skull.

Bats: temperature specialists

Many of the studies behind this research involve bats. When scientists compared natural tree roosts with artificial bat boxes, the boxes often did worse: they overheated during the day and lost their warmth quickly at night. Real cavities buffered the heat far more effectively.

Bats, it turns out, are picky about their real estate, and for good reason.

Owls: quiet occupants of borrowed homes

Some owls, including tawny, screech and boreal owls, nest and roost in natural cavities, often ones a woodpecker began. For a female incubating eggs or a chick that can’t yet regulate its own body heat, a sheltered nest with gentler temperature swings can make a real difference. Between a scorching afternoon and a cold night, a cavity takes the edge off both.

Squirrels: the flexible neighbours

Squirrels are among the most adaptable cavity users. Many also build leaf nests, called dreys, but when the weather turns harsh, they often retreat to a hollow. Even a nimble, mobile animal with other options picks the insulated shelter of a tree when conditions get extreme.

Why This Changes How We Think About Old Trees

Owls, bats, woodpeckers and squirrels don’t just need trees. They need old, imperfect, cavity-riddled trees. A smooth young trunk with no scars, splits or woodpecker holes offers little shelter.

That gives forest managers a practical takeaway: protecting old-growth and structurally complex trees isn’t only about scenery. It’s climate infrastructure for wildlife. Standing dead and dying trees, which can look untidy, are often the most valuable of all.

It’s also a caution about our replacements. Nest boxes and bat boxes are useful conservation tools, but many standard designs, especially those with thin walls, fall short of what a real cavity does. They tend to heat up quickly by day and cool down quickly at night, the opposite of what a good shelter should do. Better designs exist, and there’s plenty of room to improve them.

The Biomimicry Angle

A tree cavity achieves passive climate control with no moving parts, no electricity and no maintenance. Its likely ingredients are:

• Thick walls. The surrounding wood acts as thermal mass, slowing the transfer of heat in and out.
• Wood’s own structure. Wood is made of tiny air-filled cells, and trapped air is a superb insulator.
• A small entrance. A narrow opening limits how much outside air swaps with the air inside.
• Shape and size. A compact, enclosed chamber loses and gains heat slowly.

Compare that with a typical nest box: thin plywood walls, little thermal mass and a large surface exposed to the sun. It’s easy to see why the two perform so differently.

For human design, the lesson is the same: thick, layered, air-trapping walls do quiet work that thin, single-layer ones can’t. It’s a strategy worth studying for shelters of all kinds, from wildlife boxes to buildings.

Some questions to ponder over:

• Could nest box and bat box designs copy the layered, insulating structure of real wood instead of thin plywood?
• How thick does a wall need to be before it starts to buffer the daily temperature swing?
• Where else in nature do hollow, air-buffered spaces create stable microclimates? Termite mounds are a classic biomimicry example, and you can read more about them in Buildings Inspired by Termite Mounds.

Try This With Young Explorers

Activity: Build a “Tree Hollow” and Test It

Suitable for ages 7+. Younger children will need help reading thermometers, and an adult should supervise any use of scissors.

You’ll need: two identical small boxes with lids (shoeboxes work well), three thermometers, cardboard, scrunched paper or fabric, scissors, tape, and a notebook or printed table.

Steps:

1. Build the two shelters. Leave one box as it is (the “thin-walled box”). Line the walls, lid and floor of the other with several layers of cardboard, paper and fabric to mimic thick, insulating wood (the “tree hollow”). Cut a small hole in each lid for a thermometer, and make the holes the same size.
2. Predict. Before you start, ask the children which box they think will stay coolest, and why. Write the predictions down.
3. Set up. Put both boxes in the same sunny spot outside, or by a sunny window, with the lids on. Use the third thermometer to record the outside air temperature in the shade nearby. Keep the thermometers themselves out of direct sun.
4. Measure. Record the temperature in each box and outside every 15 to 20 minutes for one to two hours. A simple table works well: time, thin box, tree hollow, outside air.
5. Compare. Which box stayed cooler? Which changed fastest? Which came closest to the outside temperature?

Discuss: What does this tell us about why animals prefer real tree hollows to thin, human-made boxes? What else could we change to make the “tree hollow” even better, such as a thicker layer or a smaller entrance?

Extension: Repeat the test in the evening to see which box holds its warmth longer, and compare the two nights.

It’s a simple, hands-on way for children to feel the science behind the headline, and to start thinking like engineers who borrow ideas from nature.

The Takeaway

The next time you see an old tree with a knot, a split or a woodpecker hole, you’re not looking at damage. You’re looking at a climate shelter, one that squirrels, owls, bats and countless other creatures are quietly counting on as the world heats up.

Opportunities for Everyone.

Source:

Cotten et al., “A global review of the capacity of tree cavities to buffer temperature extremes,” Forest Ecology and Management, August 2026.

AI was used as a Thinking Partner.