Thank you to everyone who participated in the 12th Annual Great Wisconsin Bat Count this summer. Scientists, landowners and volunteers spent a few of their summer evenings observing bats as they left their roosts both before the newborn pups take flight and after.
This collaborative effort helps our staff better understand the populations of little brown bats and big brown bats in Wisconsin, and we’re so grateful to everyone who offered their time to help these efforts!
This year’s post-volancy (flight) count included over 120 roosts and more than 20,000 bats!
Little Brown Bats
16,865 individuals at 57 sites
Big Brown Bats
3,714 individuals at 67 sites
Last year’s totals were 4,570 big brown bats and 20,944 little brown bats, so we are down by quite a bit for both species.
However, nearly 17,000 little brown bats is still an impressive number, and we’re still waiting on a few counts at sites that may push this number up another few thousand bats. Looking back at the statewide counts for big brown bats, there’s a fair amount of variation in totals for this species, and this year, many roost monitors noted many fewer or no bats at sites. This is possibly because the very hot conditions caused them to seek cooler places as soon as the juveniles were able to fly. Big brown bats in general seem to be less tied to specific roosts and readily shift around among sites depending on what’s going on with temperature, time of year and even parasites. Many who note bats leaving in July often record them returning, sometimes in large numbers, later in the season.
As far as the decrease in little brown bats, there are a few possible reasons:
- Cold weather in May and June may have had a negative impact on pregnant bats. This may have limited food and forced them to use more energy to stay warm on cool nights.
- Pup mortality isn’t a likely factor at little brown roosts because our volunteers and landowners often report on this in addition to participating in bat counts and we didn’t receive such reports.
- Since little brown bats tend to roost in large numbers, we may be approaching the “carrying capacity” for some of these sites where the space can’t support much higher numbers of bats than we’ve seen recently.
- Large increases in prior years may have been possible because of how detrimental white-nose syndrome (WNS) was to bat populations. Reaching the endemic phase of WNS may mean more steady numbers and not as striking yearly increases as bat colonies recovered.
Overall, despite lower bat numbers, we’re still optimistic for the ongoing recovery of little brown bats and the continued health of big brown bats in the state. Thank you to everyone who surveyed bats this summer!
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