Abstract
Wind turbines represent a source of hazard for bats, especially through collision with rotor blades. With increasing technical development, tall turbines (rotor-swept zone 50–150 m above ground level) are becoming widespread, yet we lack quantitative information about species active at these heights, which impedes proposing targeted mitigation recommendations for bat-friendly turbine operation. We investigated vertical activity profiles of a bat assemblage, and their relationships to wind speed, within a major valley of the European Alps where tall wind turbines are being deployed. To monitor bat activity we installed automatic recorders at sequentially increasing heights from ground level up to 65 m, with the goal to determine species-specific vertical activity profiles and to link them to wind speed. Bat call sequences were analysed with an automatic algorithm, paying particular attention to mouse-eared bats (Myotis myotis and Myotis blythii) and the European free-tailed bat (Tadarida teniotis), three locally rare species. The most often recorded bats were the Common pipistrelle (Pipistrellus pipistrellus) and Savi’s pipistrelle (Hypsugo savii). Mouse-eared bats were rarely recorded, and mostly just above ground, appearing out of risk of collision. T. teniotis had a more evenly distributed vertical activity profile, often being active at rotor level, but its activity at that height ceased above 5 ms-1 wind speed. Overall bat activity in the rotor-swept zone declined with increasing wind speed, dropping below 5% above 5.4 ms-1. Collision risk could be drastically reduced if nocturnal operation of tall wind turbines would be restricted to wind speeds above 5 ms-1. Such measure should be implemented year-round because T. teniotis remains active in winter. This operational restriction is likely to cause only small energy production losses at these tall wind turbines, although further analyses are needed to assess these losses precisely.
Highlights
The continuous supply of fossil energy sources such as petrol is compromised in the long run while the global warming crisis calls for a reduction of carbon and other greenhouse gas emissions
This study provides quantitative assessments, from data collected at various heights above ground level, of the vertical activity profiles of a bat community occurring in an area where tall wind turbines are being deployed
The techniques used so far for investigating bat vertical activity include bat detectors attached onto helium-filled balloons or blimps [33,34], tethered to kites [33,35], placed on meteorological, telecommunication or flux towers [36,37,38], installed in the tree canopy with a pulley system [39] or put on the jib of a crane [40]
Summary
The continuous supply of fossil energy sources such as petrol is compromised in the long run while the global warming crisis calls for a reduction of carbon and other greenhouse gas emissions. There are several non-mutually exclusive explanations why bats collide with wind turbines [17] They may fail to detect the rapidly approaching blades because of the extremely high rotor speed (up to 300 km h-1 at blade apex), in part due to the very focal character of bat sonar. They may underestimate blade velocity (about ten times their flight speed) when maneuvering in the rotor-swept zone, failing to avoid collision [9]. They might be attracted to wind turbine towers as such tall elements dominating the landscape might attract insects [18] or be perceived as potential roosts or even as vantage mating sites [19]
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