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Providing evidence to improve practice

Action: Automatically switch off wind turbines when bat activity is high Bat Conservation

Key messages

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  • Two studies evaluated the effects of automatically reducing turbine blade rotation when bat activity is high on bat populations. One study was in Germany, and one in the USA.



  • Survival (2 studies): Two replicated studies (one randomized, controlled and one paired sites study) in Germany and the USA found that automatically reducing the rotation speed of wind turbine blades when bat activity is predicted to be high resulted in significantly fewer bat fatalities for all bat species combined and for little brown bats.


Supporting evidence from individual studies


A replicated, paired sites study in 2012 at eight pairs of wind turbines in Germany (Behr et al 2016) found that using automated ‘bat-friendly’ operating systems that reduced turbine blade rotation speed resulted in fewer bat fatalities than at normally operated wind turbines. Total bat fatalities and average collision rates were lower at automated turbines (total 21 bat fatalities, 0.06 fatalities/turbine/night) than at normally operated turbines (total 2 bat fatalities, 0.01 fatalities/turbine/night). At automated turbines, predictive models identified periods of high fatality risk and low energy yield from bat activity and wind speed data. During these periods, rotor blades were moved parallel to the wind to reduce rotation speed according to a target bat fatality rate (0.01 fatalities/turbine/night). Normally operated turbines rotated freely. At each of eight sites, automated and normal operating modes were alternated weekly between two paired turbines over 14 weeks in July–October 2012. Carcass searches were carried out daily. If applied to all turbines, it was estimated that automated operation would result in annual energy losses of 2.1%.


A randomized, replicated, controlled study in 2015 at a wind energy facility in Wisconsin, USA (Electric Power Research Institute 2017) found that using automated ‘Smart Curtailment’ operating systems that reduced turbine blade rotation speed resulted in significantly fewer fatalities for all bat species combined and for little brown bats Myotis lucifugus than at normally operated wind turbines. There was an 83% reduction in fatalities for all bats and a 90% reduction in fatalities for little brown bats at automated turbines (all bats: average 3 fatalities/day; little brown bats: 0.3 fatalities/day) compared with normally operated turbines (all bats: 18 fatalities/day; little brown bats: 3 fatalities/day). Twenty turbines were randomly selected for the study (10 operated by automated systems and 10 normally operated). At automated turbines, fatality risk was calculated by a predictive model using real-time bat activity and wind speed data every 10 minutes. If fatality risk was high (wind speed ≥3.5 m/s and >1 bat call detected in the previous 10 minutes), rotors were slowed (to ≤2 rpm) for 30 minutes. Normally operated turbines rotated freely. Carcass searches were carried out daily at all turbines in June–October 2015. Electricity generation was reduced by 90 MWh/turbine at automated turbines during the study period.

Referenced papers

Please cite as:

Berthinussen, A., Richardson, O.C., Smith, R.K., Altringham, J.D. & Sutherland, W.J. (2018) Bat Conservation. Pages 67-93 in: W.J. Sutherland, L.V. Dicks, N. Ockendon, S.O. Petrovan & R.K. Smith (eds) What Works in Conservation 2018. Open Book Publishers, Cambridge, UK.