In recent years, biodiversity has declined faster than ever before in human history. Appropriate monitoring methods are needed to detect the decline of populations in time to be able to take conservation measures. Information on population dynamics can only be obtained by conducting standardised monitoring programmes, and the quality of the data depends on the survey method used. The present study aims to provide scientifically sound recommendations for the selection of a suitable survey method for emerging flights at greater mouse-eared bat (Myotis myotis) maternity roosts. For this purpose, three survey methods, namely infrared video recordings, counting by light barriers and visual counting of emerging bats, were used simultaneously for 4–5 nights at three different maternity roosts in 2019 and comparatively evaluated. Besides the quality of the counting data, the requirements and limitations of the different methods were compared and discussed. The results of this study showed that the number of emerging flights detected with all three survey methods were closely correlated, regardless of location, number of emerging flights per night (between 300 and 800) and season. Furthermore, it was shown that the presence of the observer and infrared video recordings had no significant influence on the emergence behaviour of the bats recorded by light barriers concerning the time of emergence, the duration of activity and the number of counts. As the three methods differed with regard to additional parameters, such as the need for technical equipment or qualified personnel, the time required, the costs and the error-proneness individual settings and requirements should be taken into account when deciding about the method used. However, for the continuous monitoring of greater mouse-eared bats at their maternity roosts, which is part of the monitoring requirements under the European Union Habitats Directive, the use of light barriers proved to be the method of choice, in particular as it allows to promptly detect changes in the bats’ activity as a basis for conservation measures.

In order to assess the consequences of climate change and evaluate its impacts on wildlife, it is essential to do so on a species-specific level. It is assumed that changes in the ambient temperature influence energy consumption as well as food availability and thus foraging behavior, reproduction, survival, and therefore population dynamics in bats. Based on this assumption, the present study aims to gain insights into the roosting and breeding behavior of the greater mouse-eared bat (Myotis myotis) in relation to changes of the ambient temperature. For this purpose, we investigated the effect of ambient temperature on the phenology of the greater mouse-eared bat by using activity data of the bats collected using light barriers at the maternity roosts.The light barrier used in this study is a system that detects the interruption of two light beams, for example, by a flying bat, and displays it as an electrical signal.The investigations have shown that 1. the higher the winter temperatures, the earlier the greater mouse-eared bats returned to the roosts to form the maternity colony; however, this was only true for