Evaluation of Methods for Monitoring Long-term Population Trends in Cave-roosting Bats

Because numerous cave-roosting bat species are experiencing population declines, especially those affected by the white-nose syndrome epizootic, it is essential to establish rigorous monitoring protocols to accurately track population trends over time. We tested the efficacy of low-cost visual counts to effectively monitor population trends of southeastern myotis (Myotis austroriparius) at a maternity-roost in southwestern Georgia. We conducted visual counts during evening emergence events using white light illumination. Visual counts were made during a 1-minute period out of every 5-minute interval throughout the entire emergence duration on three consecutive nights during late-June and early-July 2008 and 2009. We simultaneously recorded emergences using a night-vision video camera to allow direct comparison of visual counts with actual bat emergence numbers. Visual counts were inaccurate (F = 26.57, P < 0.0001) and inconsistent between years (F = 37.50, P < 0.0001) in providing estimates of total emergence numbers. However, depression of the emergence rate (number of bats leaving per minute) during white-light illuminated visual observations influenced our visual estimates. Additionally, we detected a positive relationship between emerging bat numbers and corresponding observer error (r2 = 0.9127, 81 d.f., P < 0.0001). Although this strong relationship suggests that potential exists to calibrate observer error associated with visual observations, we conclude that visual counts, particularly with white-light illumination, offer little merit as an effective low-cost technique to monitor southeastern myotis colonies, and may be problematic for monitoring other colonial bat species as well. Future efforts should focus on video recording methodologies to establish monitoring programs for cave roosting southeastern myotis.

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