Better Bat Boxes

On a still summer night at Sakatah Lake State Park in southern Minnesota, the bird songs of the afternoon begin to dwindle as daylight dims. Near the lake, I hear a quiet high-pitched chatter emanating from a group of bat boxes, birdhouse-like structures mounted on tall posts. I focus intently as dark silhouettes begin to drop out of narrow openings at the bottom of the boxes. At first, intermittent individuals emerge before a stream of bats flows in bursts, and my tally quickly reaches double digits just from a single box. My eyes fixed, I aim to count each bat that takes flight; every individual is important. As night takes hold and the boxes empty, I finish my survey by admiring the bats above me, watching them home in on insects with rapid maneuvers and surprising agility.

As a mammal specialist for the Minnesota Department of Natural Resources’ Minnesota Biological Survey, I have come to deeply appreciate opportunities to observe bat colonies during summer evenings—in part because these bats are survivors. More than 90 percent of Minnesota’s cave-hibernating bats, which previously composed a large portion of the state’s bat population, have been wiped out in recent years by white-nose syndrome, a deadly bat-specific disease.

Since 2015–16, when the disease was confirmed to be in the state, I noticed increasing public interest in bat boxes as a tool to help bat populations. Concerned Minnesotans were reaching out to the DNR with questions about bat box recommendations and stories about boxes they had installed that bats were not using. Broad guidelines and untested designs meant that the boxes, while seemingly a great idea, were not always attracting bats and providing quality habitat.

This observation led me and a team of DNR colleagues to launch a project to learn how boxes can best support bat populations. What we learned helped us create a set of recommendations that aims to give bats a helping hand by providing quality, safe places to raise the next generation.

A Team, a Plan. The DNR’s Minnesota Biological Survey and the Parks and Trails division often work together to monitor bats, with Forestville–Mystery Cave and Lake Vermilion–Soudan Underground Mine state parks serving as the two largest bat hibernacula—hibernation spots—in the state. Having seen the toll of disease on hibernating bats, both MBS and park staff were highly motivated to collaborate on a project. Additionally, many state parks were already hosting bat boxes on their lands, providing rich ground for study, research, and anecdotal observations.

Staff in the Parks and Trails Resource Management Program, including Tavis Westbrook and Ed Quinn, helped spearhead the project by obtaining funding from the state’s Legislative-Citizen Commission on Minnesota Resources. I worked with MBS mammalogist Gerda Nordquist to compile information about bats in parks. Together we set out to improve guidelines for bat boxes to provide quality habitat in Minnesota.

Bat boxes are incredibly important to the health of the bat populations that use them. While hibernating bats use caves to overwinter, during warmer months they roost in a variety of habitats, from trees and rock crevices to human-made structures including bridges, buildings—and bat boxes. During summer months, female bats gather in groups called maternity colonies to give birth and raise baby bats, called pups. Born hairless and completely dependent on milk from mom, pups need very warm—but not too hot—temperatures to grow quickly. It is primarily these bat moms and pups that use bat boxes. The conditions inside a bat box can influence the health and survival of bat pups, important factors for population growth.

We aimed to study bat boxes throughout Minnesota to determine, in short, what bat moms are looking for in these important seasonal shelters where they birth and raise their young.

A Trail of Bat Stories. The bat boxes in state parks provided a good starting point for a statewide study on bat boxes largely because of the people and the lore behind them.

Each box installation initiative came from someone who felt that boxes could support bats at these locations. Sometimes it was a park manager, other times a local scout troop, other times a person in the community. Because all the installations had different origins, they provided a wide diversity of box characteristics we could examine.

With the help of park managers, resource specialists, and other park staff, we documented 138 bat boxes across 28 state parks throughout Minnesota. Success often relied on finding at least one person at the park who noticed bat boxes or by following a trail of stories about parks that had a history of bat activity. At Whitewater and Forestville–Mystery Cave state parks, which are especially rich in bat habitat, naturalists Jeremy Darst and Jake Stacken were already leading interpretive programs about bats and conducting evening bat counts.

After boxes were documented, we collected detailed information about each box such as design, color, dimensions, condition, solar orientation, and mounting method. With park staff assistance, this large task was accomplished in one summer season. 

To understand bat preferences for boxes, we then took a closer look at 55 boxes with a mix of characteristics and bat use. Through the course of three summer seasons, from April to October, we—again with a lot of help from Parks and Trails staff—were able to estimate the daily number of bats in each box.

Watching boxes and tallying numbers, we patiently waited to see what the bats would tell us.

A Matter of Choice. At the start of our project, we thought that certain characteristics would be favored by maternity colonies—that bats would prefer boxes of a particular design or placement. Past thinking around bat boxes has also assumed that bats need only a single, perfectly designed box for the summer.

Instead, our observations told a different story, something critical about how bats use boxes: They move around! The data collected during this project shifts us to a new understanding of how bats interact with boxes: Choice is key.

Because temperature is so critical to bat health, bats use roost-switching strategies to find the temperatures they need for different reproductive periods in the summer. Parks that had three or more bat boxes near each other, facing different directions, allowed maternity colonies to move around to find optimum conditions.

We also found that size matters: Bigger is better. Maternity colonies used large boxes with multiple chambers and were not observed using small, single-chamber boxes. Large, multi-chamber boxes are likely to have temperature gradients from top to bottom as heat rises, and from front to back as the sun warms the face of the box during the daytime. Temperature inside a small box, on the other hand, is likely to be uniform from top to bottom and to fluctuate more with ambient air temperature.

Large boxes also allow bats social choices by providing space for more bats or space for individuals to spread out within the box. We observed bats gathering in larger groups early in the season (150-plus bats in some boxes!) and spreading out into smaller groups in later summer. Bats adjust group sizes nightly as a social thermoregulation strategy: Larger groups create warmer conditions beneficial for pup development and growth. In late summer, bats may prefer to be in smaller groups where roost temperatures are cool enough for them to use a fat-conserving hibernation state known as torpor.

Bats, it turns out, are fussy about their roosts for a very good reason: They make for healthy communities.

It Takes a Community. In fact, one of the foremost observations from this project was the importance of community, both from the perspective of bat social behavior at these artificial roosts and from the many people whose contributions helped tell this story—from those who originally installed the boxes to the park staff working there today. It is apparent that it takes a community to install, monitor, and maintain these boxes as time goes on.

We hope insights from this project can benefit bats by providing improved conditions for box-using species that are facing precipitous population declines, like the long-lived little brown bat. These animals contribute to our human communities with their voracious appetite for insects, providing critical ecosystem services that aid forest regeneration and reducing our need for pesticides by keeping insects in balance.

Some state parks are already benefitting from our project’s findings as they maintain their bat boxes and install new ones. We hope that our new bat-box guidelines, which are described on pages 28–29 and detailed online at mndnr.gov/qr/bat-roost-guide, will be consulted by anyone who, like us, wants to keep our bats flying long into the future.

If we can support surviving colonies with quality reproductive habitat and commit to the long-term effort involved in establishing and maintaining bat boxes, perhaps the experience of watching bats emerge at last light can become commonplace once again.

Box Sweet Box
If you’re interested in helping bats by providing boxes where they can roost, it’s important to choose and place the boxes carefully. Here are key guidelines for Minnesota bat boxes from a DNR study:

• Large, multi-chamber boxes are best. Tall boxes with multiple chambers allow bats to move around and to roost in different group sizes. Boxes should be larger than 3,000 cubic inches—roughly the size of a microwave oven or a 5-gallon bucket. Avoid smaller single-chamber boxes commonly sold by hardware or garden stores and online retailers; bats tend not to use them.
• Use proven designs. Styles known as the multi-chamber nursery, the rocket box, and the Johnson box work best.
• More boxes are better. A cluster of three or more boxes of different styles gives bats more options to find suitable conditions for gathering in social groups, raising pups, and resting.
• Sun exposure matters. Boxes should face different directions with a mix of sun and shade. One box should face east to warm in the morning sun.
• Mount boxes on tall posts or buildings. Never on trees.
• Choose the right site. Boxes should be mounted in an open area near forest cover, at least 20 feet away from tree branches.
• Bats need water nearby. Bats drink by gleaning water from the surface of water bodies. Boxes should be mounted within a quarter mile of a water source.

Learn more at mndnr.gov/qr/bat-roost-guide.