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 Here’s why: Bats and their pups can overheat and die in poorly
designed or placed bat boxes, and in a warming climate, it could
happen more often.
Illinois bat ecologists Joy O’Keefe and Reed Crawford recently
synthesized the available data on bat boxes, also known as bat
houses or artificial roosts, to raise awareness of the issue and
motivate change in bat box design, marketing, and consumer
education. Their recommendations are published in Conservation
Science and Practice.
“Conservation practitioners and homeowners are well-intentioned;
they want to help bats. Inevitably, the first thing they do is
go online and buy a bat house,” says Crawford, a doctoral
student in the Program in Ecology, Evolution, and Conservation
Biology at Illinois. “But unfortunately, a lot of bat houses on
the market are small and painted dark colors, and they get hot
really quickly. They’re likely to hurt bats if they’re installed
in the wrong spot.
“We're trying to highlight that there's a lot of misleading
information out there. We want to steer people away from putting
up bat boxes as a first course of action, to instead consider
the risks and ask if there is anything we can do that’s going to
be more beneficial for bats.”
Crawford has read just about every study that’s ever been done
on bat boxes, and he and O’Keefe have conducted their fair share
of studies, as well. They and other members of the bat research
community have identified a critical temperature threshold – 104
degrees Fahrenheit or 40 Celsius – above which most bat species
are significantly heat-stressed.
Although very few studies have documented temperatures inside
the countless bat boxes currently in use across the world,
researchers have recorded temperatures as high as 142 degrees
Fahrenheit in artificial roosts. And they’ve observed bats in
natural roosts moving to avoid temperatures above 97 degrees.
In many bat boxes, bats can’t move around to avoid hot spots.
The most common design is a small, flat-panel box with vertical
slats creating one to four chambers. The boxes are often painted
or stained dark colors, and that can be a big part of the
problem.
“We know darker boxes can be, on average, 41–43 degrees hotter
than white boxes and are more likely to overheat. Yet we see
companies marketing black boxes as preferable to bats and
suitable for most U.S. climates,” Crawford says. “But one hot
summer day in a really tiny black bat house is probably going to
kill anything that's in there. So it's very dangerous.”
O’Keefe, an assistant professor and wildlife extension
specialist in the Department of Natural Resources and
Environmental Sciences at Illinois, adds, “Even our colleagues
in cooler climates have seen a number of bat deaths in
dark-colored boxes. That's why we advocate for not painting
boxes in dark colors.”
But she points out that bat deaths have also
occurred in unpainted, light-wood boxes in warm climates.
“Clearly, more intervention is needed than just the color of the
box.”
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Bats are better off in tall, four-sided bat boxes (pictured) than
many other designs on the market. Photo by Michael Durham.
Bats do like to be warm and protected when they’re
sleeping and raising their young. But when they’re seeking out
natural roosts in the wild, they choose spots that allow them some
flexibility to move when it gets too hot.
That’s why Crawford and O’Keefe say if you’re going to invest in a
bat box, large designs are better. For example, tall, four-sided
boxes allow bats to move up and down in space and around to the back
side when the front is being blasted by sun.
Researchers are also investigating novel materials and design
elements that are better temperature buffers than the usual plywood,
including wood-cement (a molded mixture of wood shavings and
cement), insulating water chambers, and chimneys.
But regardless of design and color, placement has implications that
go beyond overheating.
“Obviously, you’re going to want to avoid hanging a small, dark bat
house where it’s going to get direct sun all day, especially in
warmer, southern climates,” Crawford says. “But we’re just starting
to learn about how artificial roosts might represent ecological
traps for bats. They could be preferentially selecting the bat box
because of where it is and passing over natural habitats that would
keep them safer from predators and parasites, as well as dangerously
warm temperatures.”
O’Keefe adds, “Bat houses can also draw bats to areas that are
inappropriate. That's one of the reasons I’ve moved away from
advising people to put out bat boxes as a first step in bat
conservation. People need to think about what they’re drawing bats
to. Do they have to cross any difficult boundaries, like busy roads,
to get there?
“I encourage homeowners to think about doing something else that
could benefit bats instead, like planting native trees or
wildflowers to attract insect prey, providing clean water sources,
or leaving standing dead trees wherever it is safe to do so, as
these are natural roosting habitat for bat box users.”
The article, “Avoiding a conservation pitfall: Considering the risks
of unsuitably hot bat boxes,” is published in Conservation Science
and Practice [DOI: 10.1111/csp2.412].
The Department of Natural Resources and Environmental Sciences is in
the College of Agricultural, Consumer and Environmental Sciences at
the University of Illinois.
[Sources: Joy O’Keefe & Reed
Crawford,
News writer: Lauren Quinn] |
