Efficacy of Visual Surveys for Monitoring Populations of Talus-Roosting Bats

Abstract

AbstractWinter cave and mine surveys have been the primary method to monitor status of bat populations but they are not equally effective across regions or species. Many species of bats that roost in rock outcrops during the nonhibernation period are difficult for researchers to monitor with existing methods. Although some wildlife biologists have speculated visual surveys could be used to monitor populations of bats roosting on talus slopes, they did not know the efficacy of the method. We used standardized plot-based visual surveys to quantify presence and abundance of eastern small-footed bats Myotis leibii on talus slopes in Virginia, and studied sources of variation and error. Detection probability for talus surveys was relatively high but varied based on search effort and site characteristics. Both abundance and detection probability varied more among plots within sites than among sites or years. In trials with radio-tagged bats to study the causes of false negatives, 18% of bats roosted where surveyors could not see them, and 18% of bats were visible but overlooked due to human error. Less experienced surveyors counted slightly fewer bats than the principal investigator, we suspect because the principal investigator simply found the bats first. There also was a slight learning curve among less experienced observers. Visual surveys have strong potential to aid in the study of bats that roost on talus slopes. Talus surveys, unlike longer-established methods to monitor bat populations, provide ways to assess error. We recommend using talus surveys to monitor other rock-roosting bat species with poorly understood populations, such as many species in western North America.