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

Abstract

Winter 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 non-hibernation period are difficult to monitor with existing methods. Visual surveys for bats roosting on talus slopes has been proposed as a means to monitor populations, but efficacy of the method is unknown. 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. Inexperienced surveyors counted slightly fewer bats than the principal investigator, perhaps as an artifact of the dual-observer approach. There also was a slight learning curve among students. Visual surveys have strong potential to aid in the study of bats that roost in 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.

Differential selection of roosts by Eastern Small-footed Myotis (Myotis leibii) relative to rock structure and microclimate

“…Talus slopes and their artificial counterparts also facilitate frequent movements because they contain such high densities of potential roosts. Moosman et al (2020) recorded average surface densities of 6,268 ± 257 crevices per ha at talus sites in Virginia.…”

“…After assessing results of this portion of the study, we tracked bats daily over the life of each transmitter at Surry, Sherando, and Gathright between 2010 and 2012. From 2013 to 2018 we located roosts either using radiotelemetry or with randomized visual searches that were part of population monitoring efforts (see detailed methods in Moosman et al 2020 ). We also included data from bats encountered incidentally while moving between random plots.…”

“…Visual surveys provide information about roosts at lower costs than radiotelemetry, allowing larger sample sizes and no handling of bats. Disadvantages of using visual surveys to locate roosts include overlooking bats–although this kind of detection error was relatively low at our sites ( Moosman et al 2020 )–and loss of information on sex, age, and reproductive condition of some individuals because bats are not captured. Uncertainty over sex and reproductive status of some bats led us to categorize roosts based on a combination of information obtained at the time of capture (for radio-tracked bats or those hand-captured from roosts) and information visible from outside the roost, such as number of bats present and presence of juveniles or lactating bats.…”