Bat Monitoring in the Apostle Islands, 2015–2019

Bats across the country face climate change, habitat loss, wind turbines and white-nose syndrome (WNS), a devastating fungal disease.

Two ways to spend the winter

• Migrants. Eastern red, hoary and silver-haired bats spend only the summer in the Apostle Islands, roosting in trees and clinging to branches and bark, then fly to the southwestern U.S., Mexico and even Central America for the winter.
• Hibernators. Big brown, little brown, northern long-eared and tricolored bats stay and hibernate in caves and buildings. Apart from big brown bats, which resist the disease better, they are highly susceptible to WNS. The northern long-eared bat is also federally endangered under the Endangered Species Act.

Listening past human hearing

The park began monitoring bats in 2015, placing ultrasonic recorders on the islands. Bats call in flight to navigate and find food, and like birds they can be identified by their calls — but the calls are beyond human hearing, so special microphones and software record and identify them. Some species sound alike and any one species' calls vary, so the software is not 100% accurate; a share of the files is checked by hand on a spectrogram.

Spectrograms are used to identify a species by hand when the software cannot. Calls differ in pitch (kilohertz, left axis) and length (milliseconds, bottom axis). NPS image.

All accounted for, plus one

The six species already known in the park were confirmed, and a seventh — the tricolored bat — was recorded for the first time.

* hibernates here in winter.

Red bats (left) migrate; little brown bats (right) hibernate here, in buildings and caves. They were the two species recorded most often. NPS photos.

What the recordings showed

• Most and least common. Little brown and eastern red bats were recorded most often; big brown and northern long-eared bats least.
• Spread. Six species were found at 75% to 95% or more of survey sites each year. The tricolored bat was at only 52%–86% and declined after 2015.
• Big brown bats were heard on 71% of recording nights in 2015, then on 50% or fewer in later years. In 2015 many of their calls came from over the septic fields behind the ranger residences on Rocky and Stockton Islands; they may have been roosting in the nearby buildings.
• Hoary bats were heard on 50% to 68% of nights each year. One Outer Island site — an open wetland along a stream, excellent feeding ground — logged over 600 call files a night in 2016 and had the islands' highest hoary bat activity again in 2018 and 2019.
• Trends. Activity of big brown, eastern red, hoary and silver-haired bats looks stable or slightly rising. Little brown, northern long-eared and tricolored bats are declining — the northern long-eared from 2015 and, despite being so common, the little brown from 2016. All three are highly susceptible to WNS, the most likely cause.

What's next

When the project began, the Great Lakes region was at the leading edge of WNS, and the monitoring gave parks a baseline against which to measure change. The network is working with the NABat Midwest Bat Hub on statistical models of occupancy — the probability that a species uses an area, allowing for the fact that it is not always detected — especially for the species hit hardest by WNS. In 2023 the network handed the equipment and the monitoring to Apostle Islands staff.

Sources

• National Park Service, Great Lakes Inventory and Monitoring Network, "Bat Monitoring in the Apostle Islands, 2015–2019." https://www.nps.gov/articles/000/bat-monitoring-in-the-apostle-islands-2015-2019.htm
• Goodwin, K. R., and A. A. Kirschbaum. 2021. Acoustic monitoring for bats at Apostle Islands National Lakeshore: Data summary report, 2015–2019. https://irma.nps.gov/DataStore/DownloadFile/662356
• NPS Great Lakes Network bat monitoring: https://www.nps.gov/im/glkn/bats.htm · NABat Midwest Bat Hub: https://midwestbathub.nres.illinois.edu/
• The original's set of seven bat portraits includes photos credited to the Wisconsin DNR and others, and is not reproduced here.
• Rewritten in hubnx's own words.