It’s the long-awaited bat episode! John Latimer, KAXE’s staff phenologist, has been eager to talk with Melissa Boman, an animal survey specialist with the Minnesota Biological Survey and Department of Natural Resources.
As an avid observer of nature, Latimer has noticed a sharp decrease in the number of bats over the last few decades. He turned to Boman in an Aug. 15 interview on the KAXE Morning Show to determine which factors are causing this decline.
Bat baseline
It’s hard to track population changes if you don’t know which species are present and in what numbers. The Minnesota Biological Survey aims to solve this problem by documenting which plant, animal and fungi species are present in Minnesota. Part of the documentation includes where they live and how populations are changing over time.
Until 2016, seven bat species were documented in Minnesota: little brown bats, big brown bats, silver-haired bats, eastern red bats, hoary bats, northern long-eared bats and tricolored bats. In 2016, Melissa Boman and her Minnesota Biological Survey team captured an evening bat in Ramsey County, bringing the total to eight. (The bat was a lactating female, indicating she had pups nearby and was successfully breeding in-state.)
These bats can be divided into two groups: those that stay in Minnesota to hibernate in caves over the winter, and those that are migratory. The hoary bat (the largest bat in Minnesota, with a wingspan up to 14 inches), eastern red bat and silver haired bat are all migratory.
The evening bat’s overwintering behavior is unknown. They are not documented in bat hibernacula in the winter, but also haven’t been observed migrating long distances.
Little brown bats, long-eared bats, tricolored bats and big brown bats hibernate in-state. Most are obligatory cave hibernators and only hibernate in caves. Big brown bats are the exception, with more flexible hibernation preferences. They also hibernate in buildings.
No longer batting 1.000
White-nose syndrome is a disease that has decimated populations of cave-hibernating bats throughout North America.
Historically, caves have provided bats with ideal overwintering conditions: stable temperatures and high humidity. However, caves are also ideal locations for the illness-causing fungi to thrive, and large congregations of hibernating bats also allow the fungus to spread quickly through the population. (The fungus was believed to have been introduced from Europe, likely by cave tourists whose clothing or shoes were contaminated.)
White-nose syndrome became the worst wildlife disease outbreak in North America, killing roughly 6.7 million bats between 2006-12. By 2018, over 90% of little brown bat, long-eared bat and tricolored bat populations were wiped out.
White-nose syndrome affects bats’ energy reserves, causing them to hibernate less deeply and burn through their fat deposits more quickly. This causes starvation before they emerge from hibernation in spring. A bat the size of a nickel (such as the tricolored bat) can only pack on so much extra fuel!
While migratory bats have avoided the brunt of white-nose syndrome, many are killed by wind turbines each year.
A bat-tle with bat-tery life
The destinations of Minnesota’s migrating silver-haired, eastern red and hoary bats are unknown. Radio telemetry has been an important method of tracking bird migrations, but it presents some challenges for bat researchers.
“That’s one of the technologies that I really, really hope develops in the future to allow us to better track (bats),” Boman said.
The size and weight of radio transmitters are a major inhibiting factor. Transmitters can only weigh about 5% of the bat’s body weight, limiting the transmitter to approximately 0.3 grams (0.01 ounce) for small species to 5 grams (0.18 ounce) for larger species. A transmitter that tiny has very limited battery life, lasting only 10-12 days in Boman’s experience.
Transmitters are glued between a bat’s shoulder blades, where it will not interfere with their flight and won’t be groomed off (bats are meticulously clean animals).
Once the transmitter is attached, Boman and her team are left with the immense task of tracking the bats’ movements. When roads are unavailable, these searches must be done on foot. Radio transmitters are not like a GPS tracker. Instead, researchers must play a game of “hot and cold,” with louder beeps indicating they are moving toward the animal and quieter beeps showing they are pointed away from its location.
While general information is available for migratory bat movements, there are major gaps when it comes to state populations. For instance, silver-haired bats migrate as far as Mexico, but it’s unknown if Minnesota’s populations make the entire journey or just move to the southern U.S.
Even for bats remaining in the state, seasonal patterns of movement can be difficult to discern. A major question is where the cave-hibernating bats go after emerging from hibernation. Are particular roosting areas or summer habitats linked to better overwintering survival? Are there differences in reproduction rates between summer habitats? These are basic ecological questions that have yet to be answered.
There are also large areas of the state without cave systems amenable to bat hibernation. Instead, Boman hypothesizes they may use talus slopes. There is evidence of bats using these rocky slopes for hibernation in the Rocky Mountains, but it has not yet been observed in Minnesota. For the most part, Minnesota bats make a (relatively) short migration to the cave systems of eastern Minnesota.
Who’s (th)at bat?
In addition to radio telemetry, bat researchers use acoustics and trapping to monitor populations. Acoustic monitoring uses the unique echolocation signature of each species to identify which bats are present in the area. Little brown bats call at a different frequency than long-eared bats, for instance: while these calls aren’t detectable to our ears, the microphones enable researchers to record, slow down, examine and identify the calls.
As bat researchers (called chiropterologists) obtain more recordings, they are discovering more intricacies about how bats use sound to navigate the physical and social landscape. They’ve found bats will adjust the frequency of their calls depending on what other species are present and will make social calls that function more like bird songs.
In addition to acoustic surveys, the Minnesota Biological Survey also uses mist nets to trap and identify bats. It’s a finnicky business: in addition to having great vision and echolocation, bats are extremely agile. (Despite the common idiom “blind as a bat,” bats have very good eyesight.) Boman reports many sleepless nights spent at the mist nets, watching bats fly right at the net just to turn aside at the last instant.
Batten down the hatches — carefully!
In summer, bats occasionally roost inside buildings and attics due to the warm temperature and stable environment. The best way to prevent this is by permanent physical exclusion, i.e. closing up any entrances the bats are using. This should be done outside the months of May through August, when bats are raising pups (young bats). The DNR has more information here.
Other ways to help bats include constructing bat houses, avoiding caves and mines where bats hibernate, and protecting wetlands and forests. You can report groups of bats (or dying or sick bats) here.
Funding for this project was provided by the Minnesota Environment and Natural Resources Trust Fund as recommended by the Legislative-Citizen Commission on Minnesota Resources (LCCMR).
