Cities are a challenging habitat for obligate nocturnal mammals because of the ubiquitous use of artificial light at night (ALAN). How nocturnal animals move in an urban landscape, particularly in response to ALAN is largely unknown. We studied the movement responses, foraging and commuting, of common noctules (Nyctalus noctula) to urban landscape features in general and ALAN in particular. We equipped 20 bats with miniaturized GPS loggers in the Berlin metropolitan area and related spatial positions of bats to anthropogenic and natural landscape features and levels of ALAN. Common noctules foraged close to ALAN only next to bodies of water or well vegetated areas, probably to exploit swarms of insects lured by street lights. In contrast, they avoided illuminated roads, irrespective of vegetation cover nearby. Predictive maps identified most of the metropolitan area as non-favoured by this species because of high levels of impervious surfaces and ALAN. Dark corridors were used by common noctules for commuting and thus likely improved the permeability of the city landscape. We conclude that the spatial use of common noctules, previously considered to be more tolerant to light than other bats, is largely constrained by ALAN. Our study is the first individual-based GPS tracking study to show sensitive responses of nocturnal wildlife to light pollution. Approaches to protect urban biodiversity need to include ALAN to safeguard the larger network of dark habitats for bats and other nocturnal species in cities.

The study area was located in the district of Marzahn-Hellersdorf in northeast Berlin and consisted largely of residential and commercial areas, with a high proportion of large pre-fab buildings in the north, gradually outnumbered by smaller houses in the south (Umweltatlas Berlin 2010). 

All work was conducted under the animal care and ethics approval of the federal agency (C113-G0114/15) and the approval of the conservation agency (IE 224-OA-AS/G/1167.2). Between June and early September 2015 and between July and August in 2016 and 2017, we captured 20 male common noctule bats at natural and artificial daytime roosts in the parks ‘Parkfriedhof Marzahn’ and ‘Schlosspark Biesdorf’, two larger parks which are not or only partly illuminated by street lamps, respectively. 

We attached a single GPS logger (Robin, CellGuide Ltd., Tel Aviv, Israel; Vesper, ASD technologies inc., Haifa, Israel) to the back of each captured bat using medical latex glue (Sauer Hautkleber, Manfred Sauer Germany). In 2016 and 2017, we added a small VHF transmitter (Telemetrie Dessau, Germany) to each GPS logger to improve the relocation of bats and tags. GPS locations were recorded every 30 seconds from 30 minutes before sunset until dawn. The earliest starting time was 18:33 CEST (mean = 20:18 CEST) and the latest ending time was 2:46 CEST (mean = 23:07 CEST). We ignored GPS locations with extremely large (> 1,000 meter) horizontal or vertical positional errors from further analysis. For a first inspection, we visualised the individual flight paths in QGIS 2.14 (QGIS Development Team, 2017) and removed locations from bats resting in a roost. We interpreted large gaps (> 3min) in the recording of GPS locations as roosting, as the GPS signals were not received when bats stayed inside roosts. GPS locations were split into one or more trips per animal. We defined one trip as comprising the movement path between subsequent roosting events (as defined above). In the rare case of battery or signal failure during a trip, we still used the partly recorded trip in our analysis.