Birds and Offshore Wind Turbines
(Bird watchers) tower erected next to the offshore wind farm at Tunø Knob, Denmark, for a three-year avian study which was completed in 1997. Photograph Soren Krohn
© 1997 DWIA
Tower Offshore wind turbines have no significant effect on water birds. That is the overall conclusion of a three year offshore bird life study made at the Danish offshore wind farm Tunø Knob.
The offshore wind park has been placed in this particular area because of a very substantial population of eiders (Somateria mollissima) and a small population of scoters ( Melanitta nigra ). At Tunø Knob more than 90 per cent of the birds are eiders, and about 40 per cent of the North Atlantic population of eiders are wintering in the Danish part of the Kattegat Sea.
The Studies were conducted by the National Environmental Research Institute at Kalø, Denmark.
Eight Different Studies
The very thorough study consists of both aerial surveys, bird counts from observation towers, and observations of the spatial distribution of birds at the offshore site as well as at a similar control site in the same region.
In the three year period some eight experiments were carried out. The central experiment was a so called before-after-control-impact study. From a watch tower placed one kilometre from the turbines and from aeroplanes scientists mapped the population of eiders the winter before the erection of the turbines and the following two winters.
Declining Population
During the three year period the number of Eiders declined by 75 per cent and the number of scoters declined by more than 90 per cent. But more interestingly, the population of water birds fell in all of the shoal of the Tunø Knob and not just around the turbines. This indicated that other factors than the turbines had to be taken into account.
At the same time the area was repeatedly surveyed by divers in order to determine variations in the amount of blue mussels (Mytilus edulis) which the birds prey on.
Less Food
The amount of blue mussels showed also great natural variation over the three years. Especially the population of smaller mussels which are the eiders' preferred prey fell significantly in the three year period. With these findings in mind the scientific group concluded that the changes in size and composition of the blue mussel population could explain the variation in the number of eiders before and after the construction of the wind farm.
Safe Distance
Controlled experiments stopping the wind turbines for a certain period has been performed. In another experiment decoys was used to attract the eiders, which are very social birds.
The result of the experiment using groups of decoys at different distances from the wind farm showed that the eiders were reluctant to pass at distances of 100 m or closer to the turbines.
The on/off experiment showed that there was no detectable effect of revolving rotors on the abundance of eiders in the area. In fact the eiders - like people - apparently prefer rotating turbines (but that result was clearly insignificant).
The overall conclusion of the final two experiments were that on one hand the eiders do keep a safe distance to the turbines, but on the other hand they do not get scared away from their foraging areas by revolving rotors. Also, the eiders showed normal landing behaviour until 100 m from the turbines.
Mussels Matter
Blue mussels The prevalence of eiders in the different zones around the turbines could be fully accounted for by the relative abundance of food.
The English edition of this study "Impact Assessment of an Off-shore Wind Park on Sea Ducks, NERI Technical Report No. 227 1998" is available from NERI, the National Environmental Research Institute, Denmark.
© Copyright 1997-2003 Danish Wind Industry Association
Updated 11 February 2003
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