Published by Associazione Teriologica Italiana Volume 28 (2): 272–276, 2017 Hystrix, the Italian Journal of Mammalogy Available online at: http://www.italian-journal-of-mammalogy.it doi:10.4404/hystrix–00009-2017 Research Article Up and down: B. barbastellus explore lattice towers Tobias Budenz 1 , Birgit Gessner 1 , Jochen Lüttmann 2,∗ , Florian Molitor 2 , Kerstin Servatius 2 , Michael Veith 3 1 Gessner Landschaftsökologie, Im Ermesgraben 3, 54338 Schweich, Germany 2 FÖA Landschaftsplanung GmbH, Auf der Redoute 12, 54296 Trier-Kernscheid, Germany 3 Department of Biogeography, Trier University, 54286 Trier, Germany Keywords: wind turbines Barbastella barbastellus acoustic monitoring explorative behaviour Article history: Received: 20 December 2016 Accepted: 13 June 2017 Acknowledgements Thanks to Maya Rauber for graphical support and to all those who worked in the field. This work was supported by the Stadtwerke Trier Versorgungs GmbH and Jade NaturEnergie GmbH & Co KG. The study sponsors were not involved in the study design; collection, analysis and interpretation of data; or writing. Thanks to Richard McDowell for improving the English. Jens Rydell and one anonymous reviewer substantially improved the manuscript through their comments. We also thank Danilo Russo for editing the manuscript and his good advice. Abstract During the last decade, wind turbine construction has become an issue of paramount importance for bat conservation. Several hypotheses have been proposed to explain why bats come close enough to the spinning rotor blades to get killed, some of which assuming that bats explore wind turbines. We test the hypothesis that the forest species Barbastella barbastellus explores tall towers (lattice towers). Echolocation calls were continuously recorded over a one-year period. At two study sites we analysed temporally linked consecutive echolocation recordings between neighbouring auto- mated acoustic devices (batcorder) which were installed at 3.5 m (ground), 20 m (canopy), 35 m (above the canopy) and at 50 and 80 m (open airspace). We assigned 7–10% of all contacts to ver- tical movements. Bats moved along the lattice towers at heights of between 3.5 and 20 m at both sites and between 20 and 35 m at one site. Although the extent of this explorative behaviour may have been underestimated due to limited acoustic detection distance, and although we used a lattice tower (most wind turbine monopoles are made of a different material), the almost complete lack of echolocation calls above 50 m at the first study site and above 20 m at the second study site makes it unlikely that explorative behaviour may expose B. barbastellus to significant risk. Introduction Bats are the most endangered mammals threatened by wind energy de- velopment (e.g., Rydell et al., 2016; Arnett et al., 2016; Voigt and King- ston, 2016): despite the use of a sophisticated biosonar, collisions still occur because the tips of rotor blades spin too fast (up to 300 km/h) for echo perception (Kunz et al., 2007). Mortality often reveals sea- sonal patterns: for instance,the majority of fatalities in the Northern Hemisphere occurs during late summer and early fall (Arnett et al., 2016). In 2003 and 2014, bats belonging to 27 out of the 37 species known for Europe were shown to be killed by wind turbines (Rodrig- ues et al., 2015), with Pipistrellus pipistrellus, Nyctalus leisleri, Pip- istrellus nathusii and Nyctalus noctula being predominantly affected. While the latter three are known to migrate over long distances (Popa- Lisseanu and Voigt, 2009; Dondini et al., 2013), P. pipistrellus is a loc- ally abundant, resident species (Rydell et al., 2016). Even rare species, such as Barbastella barbastellus and Rhinolo- phus spp., are occasionally found dead at wind turbines, albeit in lower numbers. In Europe, five fatalities involving B. barbastellus have been recorded (Rodrigues et al., 2015) — two in Germany and Spain, and three in France. However, such numbers have to be interpreted care- fully since almost all published bat fatalities have been recorded from open landscapes, where the majority of wind turbines were built, and not from forests where species such as B. barbastellus predominantly occur (Russo et al., 2015). It is currently unknown why and how resident forest bats come close enough to spinning wind turbine rotor blades to get killed. Several hypotheses have been put forward which are not mutually exclusive (Szewczak and Arnett, 2006; Kunz et al., 2007; Cryan and Barclay, ∗ Corresponding author Email address: jochen.luettmann@foea.de (Jochen Lüttmann) 2009; Rydell et al., 2016; Cryan et al., 2012, 2014), i.e.: (i) bats may mistake the monopole towers of wind turbines and other tall structures for potential roosts (roost attraction hypothesis); (ii) tree-roosting spe- cies may seek shelter or mating roosts in tall trees (tallest-tree hypo- thesis / roost attraction hypothesis / reproductive landmark hypothesis); (iii) landscape modifications may guide foraging bats towards wind tur- bines (linear corridor hypothesis / landscape attraction hypothesis); (iv) bats may be attracted to prey insects that accumulate around turbines (feeding hypothesis); (v) sound (including ultrasonic noise) produced by wind turbines may attract bats (sound hypothesis). To the best of our knowledge, none of these assumptions has been confirmed. Most of these hypotheses directly or implicitly assume that wind tur- bines attract bats and cause explorative behaviour. Some observations using thermal infrared imaging of flight activity of bats at wind tur- bines indicate that bats fly and forage in close proximity to wind tur- bine rotors (Ahlen, 2003; Horn et al., 2008). A recent study based on 3D reconstruction of infrared images of presumably aerial hawking bats active near a wind turbine nacelle showed movements that could be interpreted as explorative behaviour (Behr et al., 2016). However, it is still unclear whether wind turbine monopole towers in fact induce exploration flights among bats, guiding them upwards and eventually bringing them close to the rotor blades. We here investigate whether flight activity of Barbastella barbastel- lus occurs high above the forest floor or even above the forest canopy as a consequence of vertical movements of bats up and down wind tur- bine monopole towers. We conducted a 1-year acoustic monitoring survey at two 83 m high lattice towers (as surrogates for wind turbine monopoles) placed inside forests. We used a vertical setting of con- secutive automatic ultrasound recorders to detect temporally adjacent echolocation signals of B. barbastellus at two neighbouring recorders which would indicate vertical flight movements up or down the towers. Hystrix, the Italian Journal of Mammalogy ISSN 1825-5272 29th December 2017 ©cbe2017 Associazione Teriologica Italiana doi:10.4404/hystrix–00009-2017