Considering Artificial Roosts as Bat Habitat

Due to their unique abilities and nocturnal nature, bats spark our curiosity and delight. They’re awesome allies due to their prowess as hunters of pests like mosquitoes and corn earworm moths. Of Illinois’ 13 resident bat species, two are state threatened, four are state endangered and three are both state and federally endangered. Therefore, bat conservation is of interest to public and private landowners who want to help our night-flying friends. One conservation option that is often discussed is the use of artificial roosts to boost bat populations.

What are artificial roosts and why do bats use them?

Artificial roosts, most often in the form of “bat boxes,” are surrogate roosts people provide for bats. They are a popular conservation and mitigation tool used to attract bats to certain landscapes and provide them with a safe refuge. Bat boxes are typically wooden structures like bird boxes, but with an opening at the bottom and multiple chambers to accommodate large colonies. In Illinois, bat boxes provide habitat for bats naturally adapted to roosting under sloughing bark or in cavities and crevices of dead or damaged trees. Illinois bats most likely to use boxes include big brown bats (Eptesicus fuscus), little brown bats (Myotis lucifugus) and evening bats (Nycticeius humeralis). In some areas, the federally endangered Indiana bat (Myotis sodalis) also uses artificial roosts, including bat boxes.

Boxes are attractive to bats when installed in roosting or foraging hotspots, which bats frequent during the warmer months of the year. Bats are incredibly faithful to such sites, returning day after day and year after year. Bats are most likely to use boxes near favored roosting areas. When a box is new, bats may check it out while they’re foraging. Often just one bat will test the box initially. Because bats are highly social animals, they’ll share information about spacious and warm boxes with their friends, and this may lead more bats to use those boxes over time. Boxes that receive a lot of sunlight are liked best because they’re warmer, and bats also prefer boxes higher above ground that offer sufficient height for them to “catch air” as they drop into flight each evening.

How do we measure bat box success?

A common mindset is that “more is better,” so if a lot of bats are using a bat box this might be seen as a sign of success. However, research has shown that animal density is a misleading indicator of habitat quality. Consider that a lack of suitable alternatives might lead a lot of bats to aggregate in a single box. For example, when a bat box is installed in an urban area with few trees around, many bats may use the roost for lack of better options.

But even when trees are abundant, bats may form large groups in boxes. Bats like to roost in groups because when they cluster or “cuddle” their collective body heat warms up the roost, creating a more comfortable environment for them and their pups. Some very large artificial roosts may host colonies numbering in the hundreds or thousands. Colony members may think they’ve hit the jackpot and choose such roosts over trees, even when doing so could pose some risks to individuals or the group.

Ponder what we’ve learned about the value of social distancing over the past few years – perhaps bats should also be wary of large gatherings! More reliable measures of habitat quality are the health and fitness of resident animals. A healthy bat is free of disease and in good condition, whereas a “fit” bat is one that produces healthy offspring capable of producing their own offspring.

Concerns about artificial roosts

Unfortunately, bat boxes are not always effective as conservation tools. Sometimes boxes fail to attract bats, but even used boxes are ineffective when they’re poorly maintained or dangerous to their occupants.

Boxes that fail to attract bats are often too small, poorly placed (e.g., in the shade), or in undesirable habitats lacking sufficient foraging resources. It’s vital to monitor and maintain used bat boxes as these structures deteriorate over time and may be occupied or degraded by other animals such as wasps and squirrels. Choosing to install artificial roosts for bats should not be just a short-term project but rather a long-term commitment.

How can a box be harmful to bats? Because boxes are much smaller than the trees they’re meant to replace, they tend to have very different temperature profiles. Whereas tree cavities buffer outside air temperature extremes, most bat boxes do not. On a cool night in spring or summer, the inside of a box will be as cold as outside, which means pups are chilly while mom is out foraging. And on a warm, sunny day during the summer, the same box will intensify the outside air temperature, especially if painted a dark color and receiving full sun in the late afternoon.

Researchers around the world have shown that many artificial roost designs reach lethally hot temperatures on warm, sunny days. Bat boxes also concentrate bats in one location for long periods of time. Because natural roosts are ephemeral dead or damaged trees, bats switch tree roosts every few days. But in more stable structures, bats will stay put for days to weeks, making them more vulnerable to parasites, like bat bugs (Cimex spp.), and more conspicuous to predators like birds. In one study in Norway, crows (Corvus spp.) plucked bats right out of the entrances of bat boxes on the side of a building.

What are the best scenarios for installing a bat box?

Bat boxes could be useful if you wish to provide an alternate roost for bats that have been using a building. However, installing a box alone will not convince bats to leave a building, which offers more space and temperature options. Bats are voracious eaters, sometimes foraging 8 hours a night, so bat boxes should only be deployed where there is sufficient foraging habitat (forests and waterways) nearby. Bat boxes might be a good way to offer a bridge to future suitable natural habitat. For example, if a preserve dominated by 40-year-old trees offers few roosting options for bats but plenty of space to forage, boxes could be installed to offer roosting spaces while those trees mature and develop suitable nooks and crannies. However, artificial roosts are unnecessary in places with abundant live and dead trees for bats to choose from.

Making a box by hand allows for design improvements, which is crucial. If you build a bat house, I recommend using high-quality, untreated solid wood to ensure the box lasts a long time. Construct a tall box (3 feet or more) with multiple ¾-inch chambers to offer bats ample room and a vertical gradient of temperatures to choose from. Ensure the box has a small entrance (¾-inch) to keep predators out. Don’t use mesh inside but create footholds for bats using a hammer or other tool to scuff up the wood or create grooves inside.

Choose an appropriate color and use latex-based paint on the outside. For Illinois’ climate, a 20–40 percent dark box (black is 100 percent) is best, though a larger, insulated box might be safe if painted a darker color. Orient the box south or east for optimal internal temperatures. Ideally, boxes should be deployed on metal poles to keep snakes and raccoons away, with the bottom of the box at 12 feet or, ideally, higher. While tree deployments aren’t advised, placing boxes near trees will give emerging bats immediate access to safe foraging habitat. Avoid bright lights, which make bats more vulnerable to predators.

Box size, insulation, color and placement work together to decide internal temperatures. Before offering a box to bats, use a thermometer to measure its internal temperatures in the place where you wish to install it. Tweak the design, as needed, so that the top of the inside of the box is no more than 89°F on a warm, sunny day.

How to provide natural roosts for bats

When trees are present but not suitable, another option is to help trees to be good roosts. You can use a chainsaw to girdle a few medium to large live trees. Girdling kills the tree and, in 1–5 years, the bark will begin to slough off and then bats may use the tree as a roost.

Where trees are not present or abundant, plant them! Fast-growing species like cottonwoods and maples may become suitable bat roosts in just a few decades. I recommend planting some of these fast growers alongside mast-bearing trees with more longevity, like oaks and hickories. Think long-term — you are providing habitat for generations of bats.

In addition to planting trees, you’ll also be helping all bats when you plant native vegetation that can support healthy and abundant insect populations. All bats need clean water, which also boosts insect populations, so keeping streams clean and flowing is vital.

Ultimately, simply protecting and creating natural habitats will provide the most benefit for our night-flying friends.

---

Joy O’Keefe is an assistant professor of human-wildlife interactions and wildlife extension specialist at the University of Illinois Urbana Champaign. Her lab tackles many applied questions related to bat conservation, working closely with state and federal agencies. For the past 10 years, her lab has conducted numerous studies to advance management practices for the use of artificial roosts for bat conservation.