Bat Hotels Open for Business

Endless summer is over, the air is crisp, and our Vineyard bats might be starting to think about hunkering down for the winter. This means it is time for us to get their underground winter hotels ready for occupancy.

Our collaborative endangered bat research from 2016 through 2018 discovered a previously unknown behavior in northern long-eared bats (Myotis septentrionalis) on Martha’s Vineyard, Nantucket, and Long Island. We found them hibernating in crawl spaces and unfinished cinder block basements beneath older homes. Prior to this work, bat biologists thought that all northern long-eared bats migrated to the mainland to hibernate in caves and mines. Learning that they were hibernating locally was astounding, especially because the species was persisting on the islands amidst the white-nose syndrome crisis that had killed millions of bats in the Northeast. However, many older homes on these islands were being demolished and replaced with new houses that are energy-efficient but don’t accommodate bats under the structure. To address this winter housing crisis for these endangered bats, we received a U.S. Fish and Wildlife Service White-nose Syndrome Research for Conservation Grant in 2022 to build and test designs for bat hibernacula on these islands. In the fall of 2022, we constructed an underground bat hibernation site (hibernaculum) adjacent to our headquarters, using a design by Carl Herzog (a retired New York state conservation biologist). We also modified two other existing structures on Martha’s Vineyard that were similar to caves.

Playing recordings of northern long-eared bat greatest hits calls, curated by Dr. Samantha Hoff (a partner on our project), we ran lures in fall 2023 and 2024 for 14 nights to attract bats to these sites. We also placed an acoustic detector inside and outside each hibernaculum to record bat calls. To monitor temperature and humidity inside each hibernaculum, we installed HOBO data loggers. Automatic cameras were also place at each entrance grate to document any bats entering or exiting the site.

While our acoustic detectors outside these hibernation sites detected northern long-eared bats, we did not record any bats inside until November 2024 at an underground site we modified in West Tisbury near the head of a pond cove. More calls were detected inside during December 2024, then periodically January through April 2025. We were even more excited to get a photo of a northern long-eared bat at the entrance to this site on our automatic camera trap: on the evening of February 23, 2025, the camera captured a small bat emerging shortly after the acoustic detector inside had recorded calls of a northern long-eared bat! Dr. Hoff reviewed all of the acoustic data (many thousand bat acoustic files) from detectors inside and outside each hibernaculum. All hibernacula had northern long-eared bat activity recorded on the detector outside the entrance in the fall. This means that bats were attracted to the hibernation sites and investigated them. The detectors we used inside the hibernacula confirmed big brown or eastern red bats inside the BiodiversityWorks HQ hibernaculum as well. The picture of the northern long-eared inside the West Tisbury cave, however, was the first picture we have of a bat emerging from one of these sites. It was an encouraging development in this conservation effort.

An important piece of this research is to make sure we are creating a safe space for bats to hibernate where they can survive infection with the fungus Pseudogymnoascus destructans (Pd), which causes white-nose syndrome (WNS). Both northern long-eared and little brown bat (Myotis lucifugus) populations plummeted after the fungus was accidentally introduced to North America from Europe in 2006. Little brown bats are demonstrating better resilience and recovery than northerns, but northerns are persisting in small numbers in coastal areas. We believe this is because their WNS infection period is shorter due to milder winters in coastal environments where they are hibernating in or under anthropogenic structures. The Pd fungus thrives in the cool, humid spaces where hibernating bats spend the winter. When bats move between hibernation sites, which they often do, they move the fungus with them. The fungus has rapidly spread across North America. If infected bats survive the winter, they can clear themselves of the fungus after they emerge from hibernation. Once they become active and start feeding, their immune systems become active against the fungal infection. They also shift their roost preferences to warm and dry places during the spring and summer, which limits the fungal growth. However, bats can be re-infected by the fungus when they return to the damp, cool hibernation sites, where the fungus persists year-round.

Every year, whether or not we document bat activity inside or outside the hibernacula, we will "clean" the hibernacula prior to the hibernation season. We treat each hibernaculum with ultraviolet (UV) light to kill any fungus inside. UV lights are used to quickly sanitize hospital rooms and other commercial settings. Bat biologists had tested this method and found it is an effective way to clear more than 95% of Pd in experimental settings. We systematically move the UV light around the hibernacula, ensuring all crevices and corners are exposed for the appropriate amount of time (15 minutes for each area). We just finished this year's procedure, and now the bat hotels are open for business. We will place cameras and detectors at all our sites to monitor activity throughout the winter.

This fall, we also teamed up with the Martha's Vineyard Land Bank (MVLB) to treat an old root cellar foundation on one of their properties to be certain no fungus is present when their bat hotel opens for occupancy. MVLB modified this old root cellar into a bat hibernaculum after seeing examples of the hibernacula we were monitoring. Rather than demolishing this small foundation from an old farm building, MVLB retained it, added a simple roof, and made an entrance to simulate an entrance like many we tracked bats to during our research. We treated this new space with UV light, placed a HOBO reader to monitor temperature and humidity, and cut cracks into the mesh lining of the grate entrance to officially open the site for bats. In fall 2026, we may spend time at this site luring the bats in with northern long-eared bats greatest hits. Stay tuned over the winter and spring to hear if any these sites have any batty guests.