National Park Service


Date of this Version



Natural Resource Report NPS/DETO/NRR—2018/1654 / NPS 109/145393, May 2018: vi, 30 pages

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Please cite this publication as:

Hammesfahr, A. M., and R. E. Ohms. 2018. Winter bat activity in a landscape without traditional hibernacula. Natural Resource Report NPS/DETO/NRR—2018/1654. National Park Service, Fort Collins, Colorado.


United States government work. Public domain material.



Prior to 2014, bat research at Devils Tower National Monument (DETO) focused on bats present during the summer months. Biologists at DETO assumed local bats were strictly summer residents due to the presumed lack of typical habitat features associated with bat hibernation, such as caves and mines. This lack of traditional hibernacula features at DETO discouraged staff and research cooperators from studying winter bat populations. Despite the earlier assumption that bats were unlikely to hibernate on the monument, DETO documented significant winter bat activity through passive winter acoustic monitoring. This study is the first study at DETO that documents such activity. Across the northwestern United States, existing research indicates that traditional western hibernacula, such as caves and mines, support small numbers of bats (Hendricks 2012). By contrast, in the Eastern and Midwestern U.S., it is common for some caves and mines to be used by hundreds or even thousands of bats (Tuttle 1991). Where most western bat species overwinter is not well understood or documented. Inspired by both curiosity and the acknowledgement of winter bat activity in the nearby Black Hills Ecosystem, DETO biologists asked a simple question, “Are bats here during the winter, and if they are not, when do they migrate away from the monument?”

The threat of the devastating fungal infestation causing the bat disease known as white-nose syndrome (WNS) looms over every landscape that is currently presumed WNS-free, such as DETO. To better understand winter occupancy and behavior of bats at DETO, we carried out passive acoustic monitoring and emergence surveys. Acoustic detectors were deployed from mid-September 2015 to May 2016. Data from both the fall and spring were included in this report, because those periods are typically associated with important bat life events, such as swarming and mating during fall, and females gathering at maternity sites during spring (Schaik et al. 2015; Frick et al. 2010). Bats call not only to acoustically orient (echolocation), but also in social contexts; bat detectors used for detecting species presence and activity can also record social calls and bursts of activity if they occur in any season (Pflazer and Kusch 2003). Bursts of acoustic activity and social calling at a site can indicate that site is important to overwintering bats. Through this work we sought to determine if bats were present throughout the winter and, if so, characterize what types of activity were ‘typical’ for DETO. Throughout this study, we observed the greatest activity during the fall from September to mid-October, and during spring emergence in mid to late April. Throughout the entire study, we recorded a variety of complex social calls and possible feeding buzzes at varying times during the night. Most of the bat activity occurred shortly after sunset. Some of the activity during the coldest months, December through February, was rather intriguing--we recorded nearly 150 bat passes per site per month, indicating a fair amount of activity. We documented a variety of species during the winter months, identified as November through March, including several species of Myotis, as well as Eptesicus fuscus and Lasionycteris noctivagans. Based on the winter acoustic data, we believe that bats are hibernating in the Tower rock feature and/or the talus boulder field during the winter months.