Bats at Voyageurs National Park: Monitoring 2015–2019

The hoary bat wasn't known in the park until monitoring found it at every site, every year; it was the most recorded species in 2018 and 2019. USGS / P. Cryan.

Bats across the country face climate change, habitat loss, wind turbines and the deadly fungal disease white-nose syndrome (WNS). From 2015 to 2019, the Park Service's Great Lakes Network recorded bats at Voyageurs National Park in northern Minnesota.

Migrants and hibernators

• Four species live here only in summer, wintering in the southwestern U.S., Mexico and even Central America.
• The rest hibernate here in caves or buildings. Three — little brown, northern long-eared and tricolored bats — are highly susceptible to WNS; the big brown bat hibernates too but has shown resistance.

Table 1. Bat species known at Voyageurs before monitoring began in 2015 and after five years (2019). * = hibernates here.

What was known before

• In the park: only three species — little brown, northern long-eared and silver-haired. A 2010 survey of five park buildings found only little brown bats, with several likely maternity colonies.
• Nearby: in the 1980s, the Minnesota DNR found big brown, eastern red and hoary bats in St. Louis and Koochiching counties, next to the park. The Minnesota Biodiversity Atlas lists hoary bats collected in both in 1966, and big brown bat calls were recorded at the Soudan Underground Mine near Tower, about 50 miles away.
• 2015–2017: the DNR and the University of Minnesota Duluth's Natural Resources Research Institute netted six species in St. Louis County and recorded the same six in both counties — plus possible tricolored and evening bats.
• Evening bats reach only to about Minnesota's southern border, so they're unlikely here. Tricolored bats come closer: they hibernate in the Soudan Mine and have been recorded at Hovland and at Palisade Head near Silver Bay.

Listening past human hearing

Bats call in flight to navigate and find food, and species can be told apart by their calls, as birds are by song. But the calls are ultrasonic, so recording needs special microphones and software — which isn't 100% accurate, because some species sound alike and calls vary. A share of recordings are checked by hand on a spectrogram.

Spectrograms let biologists check calls the software can't identify, comparing frequency (kilohertz) and duration (milliseconds). NPS.

Seven species

• Reconfirmed: silver-haired, little brown and northern long-eared bats.
• Added: big brown, eastern red, hoary and tricolored bats, by software and hand review. The first three were expected — the park is well inside their ranges. One hand-verified tricolored recording suggests it's at least occasionally here.
• Most recorded: the little brown bat in 2015–2017, then second to the hoary bat.
• Silver-haired bats made up only 5%–7% of recordings in the first three years, then became more common. Hoary and silver-haired were the only species recorded at every site, every year.
• Trends: big brown, hoary and silver-haired look stable or slightly rising; eastern red, little brown, northern long-eared and tricolored are declining.

What comes next

The network is working with the NABat Midwest Bat Hub on models of occupancy — the probability a species uses an area, allowing for the fact that it isn't always detected — especially for the species hit hardest by WNS.

When the project began, the Great Lakes were at the leading edge of WNS, and monitoring gave parks a baseline. In 2022 the network handed bat monitoring to the parks; further work at Voyageurs is up to park biologists.

Sources

Based on "Bat Monitoring at Voyageurs National Park, 2015–2019," Great Lakes Inventory and Monitoring Network, National Park Service; a work of the United States government in the public domain. The full report: Goodwin, K. R., and A. A. Kirschbaum, 2022, Acoustic monitoring for bats at Voyageurs National Park: Data summary report for 2015–2019, Natural Resource Data Series NPS/GLKN/NRDS—2022/1342 (download). The source's picture of seven bats is left out: it includes Wisconsin DNR photographs.