Before you slow down your turbine, the animal will fly 500m in any direction

posted on April 17, 2019


Periodic shutdown of selected turbines during the highest risk periods is today the best mitigation means to avoid bat collisions (Arnett et al. 2011).

I know few common methods of shutdown:

    • hard shutdown in defined period (e.g. from sunset to sunrise)
    • calibration of higher start-wind speed, when the wind speed is below this level the wind turbine is off;
    • allowing the minimum speed, when the real wind speed is lower than critical level (feathering).

The critical wind speed for both are between 5m/s (Hein et al. 2014) and 7,3 m/s (Arnett et al. 2013). Annual productivity losses are minimum 1-3%.

However the highest bats activity period is well recognized in Europe and Nord America, the birds activity is more stochastic and practically independent of wind speed. For some bats species during migration time, the wind speed is not limited factor and wind speed correlated turn-off is not right method to minimize the impact (Hurst et al. 2014). The question of the efficiency of mitigation is today one of most important for EUROBATS (2015).

I remember a graph of wind turbine response time after alarm, presented by Kaj Skov Nielsen at Conference on Wind energy and Wildlife impacts in Lisbon  (Nielsen 2017). The turbine needs 32 seconds to slow down from 90 RPM to 20 RPM. It is enough time for bat or bird to fly hundreds meters. Even if you stop the turbine because of bat or bird activity, animals can just bypass the rotor. Do you feel good, when sometimes you stop your turbine without reasons or because of excessive precaution? No.


Probably the only reasonable solution for bats and birds protection is planned stopping in periods of high activity of bats (like summer’s night hours) and birds (e.g. during hours of their soaring or display flights in breeding grounds, during migrations in the vicinity of stopover sites of migratory birds). In such case we lose the productivity.


How B-finder can optimize your feathering/turn-off management? With B-finder you can count all fatality cases on your project. The accurate results of mortality monitoring you can share with environmental regulators to give proof of real impact period, instead of pre-established turn-off time. Impact of individual turbines in the same wind farm is often different, because of many environmental indicators. Having precise results for each one turbine you don’t have to stop all turbines in the projects if only some turbines cause a lot of collisions. Having accurate information, regulators can tailor the critical periods and you can enjoy highest productivity. What’s more, B-finder can help you to prove your project, when authorities contest the location because of some environmental reasons like protected area, rare species distribution or birds’ migration roads.


If you have precise data about impact, you control the process without third parties influence, without ambiguity and without doubt. Your project is transparent for you, for authorities and for local society.


In B-finder Team we believe, that precise information from many B-finder-equipped wind turbines around the world can help us to cover the gap of knowledge and use right mitigation/compensation means. Our goal is lower impact with higher energy productivity. Before you slow down your turbine, consider B-finder as solution for you!



B-finder CEO



  • Arnett E.B., Huso M.M.P., Scirmacher M.R., Hayes J.P. 2011. Altering turbine speed reduces bat mortality at wind-energy facilities. Frontiers in Ecology and the Environment 9(4): 209–214, doi:10.1890/100103.
  • Arnett E. B., Johnson G. D., Erickson W. P., Hein C. D. 2013. A synthesis of operational mitigation studies to reduce bat fatalities at wind energy facilities in North America. A report submitted to the National Renewable Energy Laboratory. Bat Conservation International. Austin, Texas, USA.
  • EUROBATS 2015. Guidlines for consideration of bats in wind farms projects. Revision 2014. Publ. Series No. 6.
  • Hein C. D., Prichard A., Mabee T.,Schirmacher M. R. 2014. Efficacy of an operational minimization experiment to reduce bat fatalities at the Pinnacle Wind Farm, Mineral County, West Virginia, 2013. An annual report submitted to Edison Mission Energy and the Bats and Wind Energy Cooperative. Bat Conservation International. Austin, Texas, USA.
  • Hurst J., Schauer-Weisshahn H., Dietz M., Hohne E., Biedermann M., Schorcht W., Karst I., Biedermann R. 2014. When are bats active in high altitude above the forest canopy? Activity data from wind masts allows prediction of times with high collision risks. Book of abstracts, XIIIth European Bat Research Symposium, 1-5 September 2014, Sibenik, Croatia: 84.
  • Nielsen K.S. 2017. Bird and Bat mitigation. Lesson learned. CWW 2017 Conference materials.