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Bats and offshore wind turbines studied in southern Scandinavia 

Author:  | Wildlife


A pilot study 2002-2003 at turbines on land showed that certain locations in the landscape could explain some of the casualty risks and the main reasons for collisions were found. During the 2005 introductory studies and in the project 2006 on bats in offshore areas in Kalmarsund we could confirm earlier known flyways from coastal points and found an extensive activity of passing migrants but also of resident species coming from various directions to areas with an abundance of insects. Observations were made at Utgrunden and Yttre Stengrund in Kalmarsund in the Baltic Sea and in Öresund between Sweden and Denmark. The observers onboard the boats and at the coastal points where bats take off used ultrasound detectors, strong portable spotlights and at special times also thermal camera. Boxes for automatic recording of bats were used on land, were placed on the turbines, and on the boat’s cap. These methods resulted in a total of 12 354 observations of bats, 3 830 over the sea and 8 524 on land. Bats fly over the sea in winds up to about 10 m/s, a major part of the activity took place at wind speeds less than 5 m/s. Bats of 10 species were observed on the open sea and all of them were foraging at suitable weather conditions, which means calm weather or light breeze. The bats did not avoid the turbines. On the contrary they stayed for shorter or longer periods hunting close to the windmills because of the accumulation of flying insects. Hunting close to the blades was observed, why the risk of colliding might be comparable to land-based turbines. Bats also used wind turbines for resting.

Insects were collected at places and times when bats were observed feeding. Chironomids were dominating, but we also found many other flying species of other insect groups. Insects, but probably also crustaceans, were caught by bats in the water surface. Some terrestrial species occurred among the insects and spiders that were drifting in the air. At times we suppose that their origin was in the Baltic Republics or Russia. It was earlier completely unknown that many bat species, migratory and non-migratory, regularly use this food resource on the open sea far from the coasts in the late summer and early autumn.

With radar on Utgrunden’s lighthouse data on movements of the largest bat species, mainly Nyctalus noctula, could be studied. This gave data on flyways, directions, movement patterns when foraging, especially near the turbines. With the radar it was possible to measure altitude and the results showed that almost all activity took place below 40 m above sea level, while only a few cases of higher flight was recorded. Observations from boat showed that altitude was very variable according to the available insects. Bats were seen hunting from the water surface up to the upper part of the windmills.

Need of further research and developing methods is discussed in the report. An updated risk assessment is presented. A standpoint today is that areas with concentrated flyways and foraging habitats with an abundance of flying insects must be very carefully examined if new windmills are planned. The collision risk at offshore wind power parks is impossible to study as long as there are no such parks. Investigations on bats needed for environmental impact assessments are suggested.

To minimize the casualty risks at existing turbines further research is needed. Some measures to take have been discussed. In certain cases it is probably most effective to move a turbine a relative short distance because of the sometimes short edges of the flyways and also the insect rich habitats. Another method is to stop the turbine during periods of high risk. Because the accumulation of insects is the reason for bats hunting close to the blades methods to reduce the amount of flying insects at the turbines would be of interest. Methods to keep the bats away from the turbines do not exist and some such ideas might also have negative effects on other animals and also on humans.

Swedish Environmental Protection Agency, Report 5571, July 2007

Download original document: “Bats and offshore wind turbines studied in southern Scandinavia

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The copyright of this material resides with the author(s). As part of its noncommercial educational effort to present the environmental, social, scientific, and economic issues of large-scale wind power development to a global audience seeking such information, National Wind Watch endeavors to observe “fair use” as provided for in section 107 of U.S. Copyright Law and similar “fair dealing” provisions of the copyright laws of other nations. Queries e-mail.

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