On authority of the administrative district government of Freiburg and supported by the foundation "Stiftung Naturschutzfond Baden Württemberg", a study on the possible operating-based effects of wind turbines on bats were conducted between August 2004 and October 2005. The main purpose of this study was to answer the question whether, in the administrative district of Freiburg, Southern Germany, bats collide with wind turbines and to what extent, as reported in other areas nationally and internationally. The study consisted of three different types of complementary surveys: searching for collision fatalities under working turbines, the examination of collision fatalities to determine the cause of death, and observations of bat behaviour at turbines using a thermal imaging camera.
Between the end of July and the end of October 2005, searches for collision fatalities were conducted every five days at 16 selected, representative turbines located mainly in the Black Forest and its foothills of Southern Germany (altitudes between 470 and 1100 m above sea level). Additionally, two to three extra searches were conducted at 16 other turbines in the same area during the same time period. Between the beginning of April and mid-May, as well as between mid-July until mid-October 2005, eight of the turbines that had been checked in 2004 were searched again with the same search interval. Furthermore, specific experiments to determine the searcher efficiency and the carcass removal rate were carried out at selected turbines. Taking into account these error factors, the actual number of collision fatalities was projected. The following results were gained:
- A total of 50 bat carcasses were found, 45 during the systematic fatality searches and five more during extra searches. When comparing the results from the eight turbines that were searched in both years, the numbers differ significantly between the years: in 2004, 31 carcasses were found whereas in 2005, with the same search intensity, only 10 were documented.
- The species found were Common Pipistrelle Pipistrellus pipistrellus (39 spec.), Leisler's Bat Nyctalus leisleri (8 spec.), Parti-coloured Bat Vespertilio murinus (2 spec.) and one Serotine Bat Eptesicus serotinus.
- Apart from bats, only nine bird carcasses were found: three House Martins Delichon urbic), three Swifts Apus apus, one Alpine Swift A. melba, one Goldcrest Regulus regulus and one Melodious Warbler Hippolais polyglotta. Overall, five times as many bats were found.
- Average searcher efficiency in the different ground coverage classes was: 84% in open areas, 77% in overgrown areas and 40% in heavily overgrown areas. The carcass removal rate differed between the sites; the average though for all experiments was rather high. For a five-day interval the average was 58.8%.
- Considering the searcher efficiency, the carcass removal rate and an area factor describing the relation of searchable to non-searchable area in a 40 m radius, it is possible to estimate the actual number of dead bats from the number of bat carcasses found. This projection results in 335 bats for the 16 turbines regularly checked in 2004 with a variation of the carcass removal rate from a minimum of 269 to a maximum of 446 bats. This equals 20.9 bats (16.6 - 27.9) per turbine. For the turbines surveyed in 2005, a total of 95 collision fatalities (75 - 125) were projected, which equals 11.8 bats (9.4 - 15.6) on average per turbine.
- Most bats were found between the end of July and mid-August and at the beginning of September. Between the beginning of April and mid-May 2005, no bat carcasses were found at any of the eight surveyed turbines.
- The Common Pipistrelle Pipistrellus pipistrellus, the species which was mainly affected, is not a migratory species.
- All the fatalities were found at turbines either in forests or in wind throw areas, none were found in open areas.
The examination of the bat carcasses showed that some of the bats had broken wings or obvious head injuries. Other bats showed signs of skull fractures. The dissection proved that most bats had internal injuries which were beyond doubt of traumatic origin. The results of these examinations lead to the only plausible conclusion, that the bat fatalities are in causal relationship to wind turbines.
Using a thermal imaging camera, bat activity was observed at two turbine sites (one in forest, one in open area) and at a third site without a turbine (wind throw) as reference, for four half-nightly observations. Highest bat activity was recorded at the reference site. At both turbine locations, bat activity was similar at an altitude > 40 m. This is contrary to the search results, where a large number of bat carcasses were found at the forest site but none at the open area site.
Approximately 25% of the bats approaching a rotor showed evasive behaviour. A bat colliding with a rotor could not be observed with certainty. At an altitude > 40 m with wind speeds between 3.5 and 7.5 ms-1, slightly more bats - and at wind speeds higher than 7.5 ms-1 slightly fewer bats - were observed than would have been expected for the distribution of wind speeds. Bat activity could be observed near turbines at higher wind speeds of up to 10.9 ms-1.
Because of strict protection regulations for bats in the Habitats Directive (European Council Directive 92/43/EEC on the conservation of natural habitats and of wild fauna and flora) and also in the German Federal Nature Conservation Act (Bundesnaturschutzgesetz), and due to the potentially high impact risk, it is recommended to carefully review the bat conservation issues during the planning and approval procedures for wind facilities. Especially important is impact avoidance, achieved most likely through site selection. According to current knowledge, turbine sites in forests and/or on ridges should be considered as potentially very problematic.
Another possibility for mitigation is restricting the operating times of the facility when bat activity is especially high. But current data is not specific enough and at times contradictory, so that generally valid guidelines to restrict operating times in specific seasons or at certain wind speeds cannot be established. Accordingly, the only solution at the moment is to carefully examine each location during the planning process and once approved, to monitor the effectiveness of the mitigation measures.