The Effect of Wind Power on Birds and Bats - A Synthesis


Title: The Effect of Wind Power on Birds and Bats - A Synthesis
Publication Date:
August 01, 2012
Document Number: 6511
Pages: 152
Technology Type:

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Rydell, J.; Engstrom, H.; Hedenstrom, A.; Larsen, J.; Pettersson, J.; Green, M. (2012). The Effect of Wind Power on Birds and Bats - A Synthesis. Report by Vindval. pp 152.
  • The wind power industry almost certainly faces a considerable expansion within the near future in Sweden and elsewhere, and it is probably unavoidable that birds and bats will be killed or otherwise affected negatively to some extent. However, we believe that an increase in wind power production according to the national plan (30 TWh until the year 2020) is compatible with the preservation of viable populations of all bird and bat species in Sweden. The risk of negative effects can be limited considerably by planning and cooperation and by using the available information. On the other hand, there are also considerable gaps in our knowledge and these should be filled in order to minimize the uncertainties during future projects.
  • We have reviewed the existing (2010) literature on the effects on wind farming on birds and bats in Europe and North America. The information has been analyzed with respect to species and groups of species, their occurrence and behavior and also according to the location and size of wind farms and wind turbines. The identified effects may be either direct, when animals are killed, or indirect, when their habitats are changed as a consequence of the establishment or operation of wind energy facilities. The indirect effects are believed to be relatively small for bats but they are probably the most important for birds. We have not reviewed effects arising from construction of power lines, extraction of materials for construction, changed hydrology and the like.
  • A wind turbine in Europe or North America kills on average 2.3 birds and 2.9 bats per year. These are median values, however, and the variation is large (0-60 birds and 0-70 bats) and the distribution uneven (bimodal). While most wind turbines actually kill none or very few birds and bats, some turbines kill many. The location of a wind farm in relation to the local topography and surrounding habitat is the primary determinant of the number of birds and bats that will be killed.
  • By far the most important measure that can be taken to minimize the risk of negative effects on birds and bats is to identify the dangerous locations and avoid locating wind turbines there. Most accidents with birds occur in places where they concentrate, such as near wetlands and bodies of water, but sometimes also in elevated sites including peaks and ridges of hills and mountains. For bats the most dangerous locations include coastlines and the top of distinct hills, but linear landscape elements such as lake shores, rivers, motorways, and, on a smaller scale also tree-lines, hedgerows and the like should also be considered as potentially risky. In contrast, in areas of intensively managed forest or open farmland the effect of wind turbines on birds and bats are usually small, particularly in flat terrain. Most future wind farm establishments in Sweden will probably be allocated to elevated sites within any of the two major forest regions. Such locations are generally not considered dangerous for birds and bats, but recent evidence from Germany and USA suggests that wind turbines located in such places sometimes are very dangerous to bats. Unfortunately, there is no information on the reaction of bats to wind turbines at high elevation forest sites in Sweden. This requires investigation as soon as possible.
  • All flying birds may potentially collide with wind turbines. However, raptors, grouse and their allies, and also gulls and terns tend to collide more often than expected from their occurrence and numbers. Birds that breed, stop over or overwinter in a particular area, and thus spend more time there, face a higher risk to collide with wind turbines, compared to birds that pass over during migration. The fatality rate at a certain wind farm generally does not decline with time, which indicates that birds do not learn to handle the problem.
  • There is no evidence that present or planned (30 TWh until 2020) wind farming in Sweden will affect any bird population at the national level, although eagles and other large raptors, as well as some waders, could possibly be affected locally or regionally. Nevertheless, particular attention is needed in areas where raptors are concentrated and in places with higher densities of breeding waders such as coastal meadows, bird islets and some bogs and mountain locations.
  • Birds, with the possible exception of swallows and swifts, are normally not attracted to wind turbines. Instead, they either avoid or ignore such installations, and this applies both to land based and off shore wind farms. During the breeding season the disturbance range is usually short or difficult to determine, but its presence is more obvious in waders than in other birds. Furthermore, it is more obvious at other times of the year, particularly in water birds that live in flocks, including divers, geese, ducks and waders. Disturbance reactions usually become obvious within 100-500 m from the turbine, but for some birds such as divers, the distance can be longer.
  • Bird densities in areas used for wind farming may either decrease or increase with time. We have been unable to find any general trends in this respect, however, although many high quality studies have been reviewed. The same situation applies to habituation, which means that the behavioral disturbance effects may either increase or decrease with time. If the densities and behavioral effects increase, decrease or remain stable over time seems to depend on the bird species in question and the particular situation.
  • Migrating seabirds usually avoid flying close to wind turbines both in daytime and at night. In daylight, obvious changes in the flight paths occur at 1-2 km (sometimes 5 km) from the turbines, but at night the reaction becomes obvious only at 0.5-1 km. The change inflight direction may lead to barrier effects and hence longer flight paths around the wind farms. On the other hand, accidents with migrating seabirds at marine wind farms seem to be very rare.
  • Bats are killed at wind turbines as they hunt for insects that accumulate around the turbine towers. The immediate causes of death may be either fractures resulting from collision with the rotor blades, or ruptures of blood vessels or lungs visible as internal hemorrhaging. In the latter case, the damage is caused by rapidly falling air pressure behind the rotor blades. The accidents usually (90%) occur during warm nights with slow wind speed in late summer and autumn (late July to September), but sometimes also in spring (May to early June). Very few bats are killed at wind farms in the middle of the summer and during the winter season. Like bats, swallows and swifts are also killed while feeding on insects at wind turbines, but the extent of this is unknown and needs to be investigated.
  • Accidents with bats at wind farms are predictable with respect to the time of day and prevailing weather conditions and usually occur during a limited part of the year (late summer) as well. In contrast, accidents with birds at wind farms tend to occur throughout the year and without any obvious coupling to the season and prevailing weather conditions. This difference between birds and bats is fundamental and implies that the two groups of animals should be handled separately with respect to wind farming. The continued use of a wind turbine that proves to be dangerous for bats may perhaps be facilitated, providing a mitigation scheme is worked out. This seems to be more difficult to do for birds, because their contact with wind turbines is more unpredictable in general and also highly variable among species. Hence, careful consideration of the turbine location before construction is most important for birds.
  • Bats occasionally hunt migrating or drifting insects that form local swarms at wind turbines far out at sea, but if this behavior results in bats being killed at marine wind farms has not been investigated. However, the behavior of bats at offshore wind turbines is similar to that observed at wind turbines on land, so until evidence is available we should expect that the risk of being killed is also similar.
  • The risk that bats are killed at wind turbines varies strongly from species to species. For some species, fatalities are rare or occasional, while other species are much more vulnerable. The high-risk species are adapted to catch insects in the open air and include the common noctule, the parti-colored bat, the northern bat and the pygmy pipistrelle and also their rarer relatives Leisler's bat and the common and Nathusius' pipistrelles. These species together comprise as much as 98% of all fatalities of bats at north European wind farms. Other species, some of which are very common, seem to spend less time at heights where they are at risk to collide with turbine rotor blades. Nevertheless, there are a few species, notably the barbastelle, which are hard to categorize. They occur in scarce or small populations, which in itself could be the main reason why they are rarely found dead at wind turbines.
  • Taller turbines kill more bats compared to lower turbines, but this does not seem to apply to birds, perhaps with the exception of certain raptors. The modernization of older wind power facilities usually means that the turbines become higher and more efficient but possibly fewer. Hence modernization of older wind farms may result in lower risks for birds in general but at the same time, the risk for bats and possibly raptors probably increases. Otherwise, the fatality rate, defined as the number of fatal accidents per turbine and year, does not seem to be related to the construction or lighting of the turbines or to their internal location within the wind park. Likewise, we found no evidence that the fatality rate depends on the distance between the rotor and the ground or on the size of the wind farm (number of turbines).
  • To evaluate the possible impact of future wind farming on bat populations in Sweden, we developed a simple mathematical model. Unfortunately, the necessary demographic information is not available for Swedish bat populations, so we had to use data from Germany. This means that the conclusions become less reliable. Nevertheless, our modelling suggests that we should not exclude the risk that the wind farm development along the national plan (30 TWh until 2020) could have a significant negative impact on some bat species at the national level.
  • The risk that a bird or a bat is killed at a wind turbine is probably small compared to the risks faced from other human activities. However, the mortality at wind turbines is different from other mortality factors with respect to which species and age groups that are affected, and therefore the risk of potential long-term effects of wind farming on birds and bats should not be neglected.
  • An already published model (Ahlén 2010a) may be used as a general guide for the handling process of wind farm applications. Suggested localizations of turbines may be considered as either a) "high-risk", where negative effects on bats or birds can be expected, b) "uncertain", where a qualified evaluation requires pre- or post-construction surveys, or both, or c) "low-risk", where negative effects on birds or bats are considered unlikely.
  • We present what we believe should be included in a wind farm EIA (Environmental Impact Assessment) with respect to birds and bats and also how the pre- and post-constructions surveys should be carried out. To maintain the quality of the surveys, it is essential that they are made generally available and open to discussion for extended periods. Hence, the survey methods and protocols should be standardized and the results should be published or otherwise made accessible in printed form or on the internet as early as possible.
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