Can acoustic recordings of cave-exiting bats in winter estimate bat abundance in hibernacula?

Abstract

Many bat species in North America face precarious futures due to white-nose syndrome and wind energy development. These threats, and others, make it imperative to monitor long-term population fluctuations of bats. Counting hibernating bats in caves estimates abundance of these cave-dwelling mammals, and such counts can be used to understand population fluctuations across time. Passive acoustic monitoring can capture large amounts of long-term data that are standardized, scalable, and reproducible; these data can also be archived for future analyses. Little work has been done, however, assessing the relationship between recorded acoustic data from bats that arouse from torpor during hibernation and fly out of caves and number of bats counted in hibernacula. We acoustically monitored and counted two species of bats in 9 hibernacula from November to March 2011 to 2018. We hypothesized that acoustic data recorded from Townsend’s big-eared bats (Corynorhinus townsendii) and western small-footed myotis (Myotis ciliolabrum) exiting hibernacula could be used to estimate abundance of those species in hibernacula. Over seven years, we conducted 29 hibernacula surveys and simultaneously set passive acoustic detectors during winter when those surveys were conducted. Acoustic monitors recorded for 1,063 nights. Detectors recorded 2,459 files of Townsend’s big-eared bats and 9,094 files of western small-footed myotis. Across all years and hibernacula, mean number of Townsend’s big-eared bats counted in a cave was 96; mean number of western small-footed myotis counted in a cave was 7. For Townsend’s big-eared bats, the top model held 43% of model weight and included the variables bat activity (i.e., mean number of Anabat files/night) and cave. For western small-footed myotis, the top model held 55% of model weight and contained the variable bat activity. Mean number of acoustic recordings of bats flying out of a cave across a winter was positively related to number of hibernating bats counted in a cave in a year for western small-footed myotis—with greater variability for estimates with a larger number of acoustic files recorded of bats flying out of caves; for Townsend’s big-eared bats that relationship held for 2 caves with > 90 individuals. Our results indicate that researchers can use passive acoustic data as an index of abundance of bats hibernating in caves in temperate climate zones. Additionally, our data indicate that researchers can acoustically detect hibernating western small-footed myotis even when these bats are not observed during hibernacula surveys. This monitoring can minimize the potential effect of researchers on the behavior of bats, facilitate long-term monitoring, and reduce the number of occasions that biologists enter caves to count bats.