Imagine moving into a new house just to discover it has no insulation. That’s what life can be like for a bat box resident.

Bat boxes, a kind of artificial roost, are a simple and cost-effective way to increase habitat for these flying mammals. They’re a preferred spot for mother bats to raise pups if their favorite housing option, large dead trees, are hard to come by. But if improperly designed, bat boxes can end up hurting more than helping, Illinois bat researcher Joy O’Keefe says.

O’Keefe is an Illinois Extension wildlife specialist and an associate professor of natural resources and environmental sciences; both units are part of the College of Agriculture, Consumer and Environmental Sciences. In a new Ecological Solutions and Evidence study, O’Keefe, along with Indiana State University colleague George Bakken, leveraged digital simulations to discover that simple insulation changes could make bat boxes much safer.

What’s in a bat box?

Bat box design might seem straightforward at first glance. Bats need habitat, and boxes can provide shelter. But the reality is much more complicated, O’Keefe said.

“Bats can’t really evaluate the key characteristics that could dictate how healthy or safe these boxes are,” O’Keefe said. “So it’s incumbent upon us to put out good habitat for them.”

One such important characteristic is temperature. As ambient temperature and sunlight fluctuate throughout the day, bat boxes might get dangerously hot or cold. But bats can’t predict this before choosing the box to roost in, O’Keefe said, and are unlikely to leave boxes during the day due to fear of predation. The results can be deadly, as entire bat families have died in poorly designed boxes.

Improving these boxes is critical as bats face habitat loss and epidemic disease. Midwestern bats are threatened by white nose syndrome, a widespread illness caused by fungal infection, but face an even greater threat from the destruction of forests, where they live.

Insulation simulations

O’Keefe and Bakken simulated different bat box designs, varying color, dimensions, orientations, heat storage, and insulation. The team tried out pine, foam, water, and air to insulate the boxes and keep temperatures stable throughout the day. They then ran the simulations for three “days” to see how temperatures might vary.

“We’re trying to save people the energy and expense of building novel boxes with no idea how they’ll perform,” O’Keefe said. “The simulations are a good preliminary step to give you a sense of how designs will differ. The relative differences between designs should be the same in practicality as they are in the simulation.”

In the simulation, the unmodified bat box reached lethal temperatures of over 113°F (45°C) within the first day. Boxes with a heat storage layer, such as water, externally surrounded by an insulator, like foam, were cooler and retained the most stable temperature over the three days. Lighter colors and thoughtful orientation could also make the boxes safer. For instance, at low latitudes, the most effective method to provide optimal daytime temperatures was to orient the box so that its longer sides faced north and south, in addition to painting the south and west sides white and the north and east sides black.

Instead of giving out uniform recommendations — telling everyone to use pine- and water-insulated bat boxes, for example — O’Keefe and Bakken made their simulation code public, so everyone could use it. Bat conservationists can enter their latitude, box dimensions, and box color, then play around with designs until they find one that works for them. This allows practitioners to tailor their solutions to their particular climate.

Safer roosts for better harvests

Bat boxes are more than just homes for mother and baby bats. They’re an important puzzle piece in strengthening Midwestern ecosystems and reducing the pest burden faced by farmers.

Bats are insectivores — they can’t get enough bugs. They love flies, including mosquitoes. O’Keefe says we want these bug eaters around to get rid of the insects that bite us and our food, but bats need quality habitat to do that. They won’t stick around residential and agricultural areas if there’s nowhere for them to live.

O’Keefe primarily studies the Indiana bat. “It’s been endangered since the inception of the Endangered Species Act, and it relies on large dead trees,” O’Keefe said. “In one area I study, an Indiana bat population is essentially 100% reliant on bat boxes now, because there are no big dead trees on this landscape. If we hadn’t put up boxes, this colony would have moved somewhere else.”

By improving bat boxes, O’Keefe hopes to provide needed habitat for these endangered bats and other bat species. Doing so will ensure bats are foraging over our agricultural and natural ecosystems for years to come.

To learn more about safer bat boxes and how to help bat conservation, see the Human-Wildlife Interactions Lab’s website.

The study, “Simple design modifications can tailor bat box thermal conditions to life history requirements in different habitats,” is published in the Ecological Solutions and Evidence [DOI: 10.1002/2688-8319.70057]. Authors include George Bakken and Joy O’Keefe.