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.