Introduction
Bats make species-specific calls for echo-location and social functions, enabling the presence of different species and changes in their calling activity to be monitored non-invasively by passive recording and identification of their calls. Bats do, however, vary their calls in relation to different habitats, functions, and the presence of other bats. This can result in difficulties of species recognition where the calls of one species are similar to the calls of another. As a result, for some species capture and examination in the hand (or even DNA confirmation) may be needed to identify individual bats to species level (Kuenzi & Morrison 1998). Furthermore, different bat species emit different intensity or directionality of calls, and at different frequencies (e.g. Barataud 2015, Anderson & Racey 1991, Goerlitz et al. 2010), with the inevitable result that some species’ ultrasonic calls are easier to detect than others, leading to a bias in the species abundance being detected using acoustic methods. In spite of these issues, recording of acoustic activity can give a good indication of activity patterns (Beason et al. 2020).
Trapping also has inherent biases, as some species of bats are better at either avoiding traps or escaping from them than others (personal observations). Bats also differ in the height at which they fly above traps, which are typically set 1-4 m above ground level, and thus avoid them. Leon-Tapia and Hortelano-Moncada (2016) reported that in their study in Mexico, 12 species of bats were detected by using ultrasonic detectors, whereas only 5 species were trapped. Furthermore, trapping inevitably disturbs a bat’s natural activity. Indeed, trapping on successive nights leads to reduced catches, indicating some alteration in behaviour (Kunz & Brock 1975). Thus, this method cannot be utilised to observe activity accurately over small time periods, even were all species to be trapped. Given the limitations of each of these different approaches, both ultrasonic detectors and capture are needed to characterise local bat assemblages (Kunz et al. 2009).
Developments in bat detection and identification technology have enabled more detailed recording of bat echolocation calls, and the automated identification of the species emitting the call. Unfortunately, however, independent testing of these systems reveals variable effectiveness, with individual researchers’ manual identification of calls from sonograms also showing imperfect species recognition (Clement et al. 2014, Russo & Voigt 2016, Rydell et al. 2017). This is hardly surprising given the variability in call structure shown by individual species of bats in different habitats, and the inherent similarity between the calls of some species (Barataud 2015).
In Europe, species of bats within the Myotis genus (including Brandt’s bats Myotis brandtii, Daubenton’s bats Myotis daubentonii, Natterer’s bats Myotis nattereri and whiskered batsMyotis mystacinus ), as well as other species such as the barbastelle bat Barbastella barbastellus and the brown long-eared bat Plecotus auritus , aggregate at certain sites in Autumn, in a poorly-understood social activity known as “Autumn swarming” (Fenton 1969, Parsons et al. 2003a & b, Rivers et al. 2006, Glover et al. 2008). In swarming aggregations, bats appear to congregate primarily for the purpose of mating; sex ratios at swarming sites on any specific night are heavily biased towards males. Females attend swarming sites sporadically, which is believed to be in order to mate (Furmankiewicz et al. 2013, Parsons et al. 2003a & b, Rivers et al. 2006, Glover et al. 2008, van Schaik et al. 2015). Bats may attend swarming aggregations from a wide geographical area; for example, Rivers et al. (2006) found that a swarming site for Natterer’s bats had a catchment radius of up to 60 km.
Autumn swarming in these species is usually focused around underground formations (Glover et al. 2008, Parsons et al. 2003b), with common features including a well-developed underground chamber, absence of water in the chamber, and shelter (e.g. vegetation) at the entrance. There is no correlation between swarming activity and the size of entrance opening (Glover et al. 2008). Parsons et al. (2003a) reported that peak swarming activity generally occurs 6-7 hours after sunset, is reduced by rain, and is positively correlated with ambient temperature. The swarming season for Myotis bats is late summer to early autumn, with Brandt’s bats and Daubenton’s bats swarming relatively early in this period, whilst Natterer’s bats and whiskered bats swarm later in the season (Parsons et al. 2003a).
Studies of swarming bats typically involve capture of bats using mist nets or harp traps (Parsons et al. 2003a, Rivers et al. 2006, Glover et al. 2008) which is inevitably disturbing to the bats and hence can only be undertaken infrequently during any one swarming season. Trapping of bats is not a good measure of the absolute numbers present, with catch efficiencies typically being of the order of 5 - 20 % of the total number of individuals using the site on that night (Altringham 2017). Furthermore, although individual adult males can stay at the swarming site for many nights (Altringham 2017, personal observations), turnover of individuals between nights and potential trap-shyness effects lead to recapture rates of males being only 0.5 % or lower (Altringham 2017, Dekeukeleire 2017).
In addition to trapping and handling, swarming activity can be monitored much less intrusively using passive logging of bat ultrasonic calls. Glover et al. (2008) and Parsons et al. (2003a) report that there is a strong positive correlation between the number of bat echo location calls logged at swarming sites, and the numbers of bats caught. Records of bat activity in these previous studies only comprised the total number of bat passes (sequences of calls recorded by the bat detector), with no attempt to identify individual species of bat.
In the present study, the “Bat Classify” software (Scott and Altringham 2014) was used to investigate Myotis bat activity at a swarming site across the autumn swarming period, to determine whether the activity patterns of different species as measured by the classification software is consistent with the activity patterns as measured by trapping. The continual automated acoustic monitoring of bats across the active season provides much more detailed temporal information on bat activity than sporadic trapping sessions can. These combined methods were used to examine seasonal and overnight patterns of activity of each Myotis species present at a multi-species swarming site in the Wye Valley, on the border between Wales and England.