Northern Minnesota's northern long-eared bats use variety of tree species to nest in and raise their pups, but they especially favor trees that are old or starting to fail.
That's the preliminary finding of a summerlong study conducted by a team of researchers from the Minnesota Department of Natural Resources, UMD's Natural Resources Research Institute and the U.S. Forest Service.
Northern long-eared bats, which nest in trees to raise their young in June and July, don't appear to favor any single species of tree, the study found, with 206 captured bats tracked to 71 different roosts in 17 different varieties of trees.
Aspen, the most common tree in the north woods, was the most common nesting site, with 27 of the 71 roosts. Red and sugar maples accounted for 23 sites. White spruce and jackpine had a few each but no other species had more than one nest each.
"It shows they're using what's out there most. And we're going to continue to test that (finding) as we work into the fall with sound detectors," said Rich Baker, endangered species coordinator for the Minnesota DNR and a principal in the research project.
While they may not have been picky about the tree species, the bats "were most often in trees showing evidence of decline or decay," the report concludes.
Of the bats surveyed, 78 percent chose trees starting to decline or in full decay - not surprising since those trees are more likely to have hollows and other nooks and crannies for bats to escape daytime predators and the elements.
"The majority of them weren't necessarily dead trees, but they had lost a limb or something and had a little space for bats," Baker told the News Tribune.
The study also found the bats are quite communal, with an average of 21.5 bats emerging from each of the 71 tree nests discovered and a "maximum of 79 bats observed emerging from one tree."
The study is considered important as northern long-eared bats face peril from white nose syndrome, an unintentionally imported fungus from Europe that continues to kill millions of bats across the eastern U.S. and Canada.
The disease has been confirmed in Minnesota, including in the Soudan Underground Mine, although no bats have been found dead here from it yet. But bats have been killed by white nose syndrome as close as Thunder Bay, Ontario and in northern Iowa. While experiments continue to find a cure for the disease, some with promise, nothing has been found yet to stop it on a large scale.
The U.S. Fish and Wildlife Service earlier this year listed northern long-eared bats as threatened under the Endangered Species Act, giving them federal protections. Their plight makes it important to not only find a cure for white nose syndrome but also protect important habitat, such as roost trees, supporters note.
"Since WNS will likely spread to Minnesota within the next five years, obtaining knowledge about northern long-eared bat habitat use and distribution before a population decline occurs will be critical information" to reduce the impacts of the fatal disease, the study notes.
That's why scientists from several agencies fanned out across the Superior and Chippewa national forests this summer - to see which trees female bats use to raise their single pup each summer.
Researchers caught night-flying bats in nets, attached tiny radio transmitters to their backs and then set the bats free. The next day teams honed-in on the transmitters, revealing where the bats were nesting. The research focused on female long-eared bats old enough to raise pups, since leaving those nests undisturbed by logging or other activities is considered the most important step for ongoing reproduction success.
That type of favored habitat may get added protection from seasonal logging or other activities, at least during summer months. Foresters may also move to retain more old, decaying trees to help keep bat habitat available.
"The data supports the idea, and it's already being done for other reasons, that we want to leave snags and some older trees standing when we do management," Baker said.
Meanwhile research will continue this autumn as scientists use bat detectors that can separate various frequencies by bat species. They will then use the areas found to have the most bats to focus next summer's continued nest survey.
"We want to find out distribution for as many species in Minnesota as we can," Baker said. "We just have so little data on where bats are across this state."
The project is funded in part by federal agency contributions, the Blandin Foundation and the Minnesota Environment and Natural Resources Trust Fund that's stocked with a portion of the state's lottery profits.