Monitoring cave-dwelling bats using remote passive acoustic detectors: a new approach for cave monitoring

Monitoring programmes provide extremely helpful information to understand population dynamics and make effective management decisions, but they are usually constrained by methodological and economical limitations. Advances in bioacoustic technologies offer new opportunities for bat monitoring. In this study, we present a method to monitor small and medium-sized cave-dwelling bat colonies using low-cost AudioMoth passive acoustic recorders. We develop a method to convert bat passes, a measure of acoustic activity, to an estimated number of bats in the roost. To do so, we took audio-visual recordings of five Miniopterus schreibersii colonies emerging from their roosts over 25 nights. We compared the accuracy of the estimates when using site-specific acoustic data against a combined global dataset and examined the influence of the detector position on the estimates. Site-specific acoustic datasets recorded at the cave entrance showed to be adequate to estimate roost bat numbers. In addition, we acoustically monitored one cave during three months of continuous sampling to test our protocol and compared the resulting data with historical datasets of roost occupancy. This method provides a low-cost, non-invasive and simple approach to monitor seasonal and inter-year roost dynamics that can easily be implemented in long-term monitoring programmes and citizen science projects.