Abstract

1. Bat abundance, diversity and behaviour can be monitored by capturing bats for identification and measurement in the hand, but this has several disadvantages. These include disturbance to the bats, which limits the frequency with which captures can be made at an individual capture site, and potentially alters the behaviours being studied. 2. Passive recording and automated analysis and identification of bat calls offers an alternative, non-invasive approach to monitoring bats. In this study, we examine the effectiveness of acoustic monitoring in comparison with capture-based monitoring of seasonal swarming behaviour among several species of Myotis bats in southern Britain. 3. We show that both approaches have advantages and disadvantages for different tasks, but can be viewed as complementary methods for addressing different types of research questions. 4. We applied these complementary approaches, together with observations from infra-red video monitoring, to describe seasonal, overnight and species-specific variation in swarming behaviour in a multi-species community of Myotis bats. 5. In our study of swarming behaviour, capture and examination of bats in the hand was necessary for measuring sex ratios, reproductive status, and even for confirmation of species identification for some difficult to separate taxa. Capture is also an essential aspect of tagging bats for individual identification and tracking studies. 6. Passive acoustic monitoring is a valuable non-invasive method for continuous monitoring of within-night, seasonal and between-year variation in the abundance of bat calls. These can be used as an index of variation in relative abundance within -- but not between -- bat species.