A Roadmap for PIER Research on Avian Collisions with Wind Turbines in California


Title: A Roadmap for PIER Research on Avian Collisions with Wind Turbines in California
Authors: Sterner, D.
Publication Date:
December 01, 2002
Document Number: P500-02-070F
Pages: 57
Sponsoring Organization:

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Sterner, D. (2002). A Roadmap for PIER Research on Avian Collisions with Wind Turbines in California. Report by Ibis Environmental Inc. pp 57.

The purpose of this roadmap is to summarize the current status of avian-wind turbine collision research, evaluate the current knowledge of risk reduction, and provide recommendations for future research. Potential researchers can use this roadmap to help determine research priorities, and evaluators can use it to help determine whether a particular proposal meets current research goals.


Although wind power is considered to be one of the most environmentally friendly energy sources, it has been shown to cause bird and bat fatalities. In some cases, wind turbines may pose a threat to local avian populations and to certain special-status species that have been given legal protection. A recent study reported that 1,000 or more bird fatalities may occur at Altamont every year, with about 50% of these being raptors, and nearly all protected by the Migratory Bird Treaty Act, the Bald Eagle and Golden Eagle Protection Act, and/or the Endangered Species Act. Raptors are also protected under California Fish and Game Code 3503.5, which makes it illegal to “take”, possess, or destroy any raptor. The U.S. Fish and Wildlife Service considers any injury or mortality of any raptor from a collision with a wind turbine, or ancillary facilities to be a “take” and, therefore, a violation of the law. Violations can result in fines from $100,000 to $500,000.


Public perception, state and federal protection laws, and potential fines and lawsuits have resulted in delays, modifications, and stoppages of new wind energy projects in California and other states. For example, Alameda County will not approve additional permit applications to increase current electrical production (~580 MW) at Altamont Pass Wind Resource Area until significant progress toward solving the bird fatality issue is demonstrated. It is estimated that the current capacity, which is well below the existing permitted capacity of 800MW could be at least doubled.


Avian fatality studies have been conducted at several wind resource areas. In many cases, it has been difficult to compare results from one study or one site to another, because survey methods and site conditions vary. In addition, few studies have been published and subjected to peer review. Despite these problems, researchers have found that the majority of wind turbine-caused bird fatalities appear to occur in California, primarily at the Altamont Pass Wind Resource Area (WRA). In particular, some raptor species appear to be at greater risk than other, more abundant, species.


There are likely a number of factors responsible for a higher number of fatalities in California. California led the rest of the United States in wind energy development in the 1980s, before there was widespread recognition of potential bird fatality risks. The Altamont Pass WRA was built in an area with a high density of raptors and a diverse topographic environment. By accident or design, newer wind generation facilities have not been placed in areas with high-density raptor populations or the risk-related topographic diversity. In addition, newer facilities generally install a smaller number of more widely spaced wind turbines. Newer WRAs use newer-generation wind turbines that are larger, more efficient and possess other characteristics that may reduce risk as well, although these features remain untested in high-risk areas such as the Altamont Pass WRA.


Researchers have performed very few studies to determine the overall impacts of wind turbines on the population viability of an individual species. Concern has focused on the impacts of wind turbines on raptor species (particularly the golden eagle) at the Altamont Pass WRA. Although a golden eagle population model based on a seven-year radio telemetry study indicated that the population appeared stable, researchers have cautioned that continued land conversion around the WRA and wind-turbine related fatalities could affect future populations.


The causes of bird fatalities may be attributable to a number of different factors, including bird behavior, high prey abundance, turbine design, spatial arrangement of turbines, and topography. It is likely that a combination of these factors is involved. Behavioral characteristics of certain raptors, such as flying at low altitudes and focused searching and stooping for prey, may make them more susceptible to collisions. At the Altamont Pass WRA, golden eagles may be particularly susceptible, because of their particular flight behavior and because of the abundance of ground squirrels, in proximity to wind turbines. In addition, raptors and other birds may not see the blade tips of rapidly rotating wind turbine rotors because motion smear makes them appear transparent.


Several different wind turbine types can be found in California’s WRAs. It is possible that several design features, such as available perch sites, number of blades, maximum and minimum blade heights, blade velocity, rotor-swept area, and fixed versus variable turbine speed could influence the relative risk of bird collision. Researchers have found it difficult to determine the overall risk of these individual variables, as it is likely that they not only interact with one another, but also with other variables such as site characteristics (e.g. topography) and spatial arrangements.


Because of their higher efficiency and reduced average rotational and tip speeds, the newer, larger machines being proposed to replace existing turbines in California exhibit some potential to reduce fatalities. It will be necessary to rigorously monitor these turbines at the Altamont Pass Wind Resource Area to evaluate their risk in comparison to existing turbine types.


The association between spatial distribution of wind turbines and fatality rates has been evaluated at a variety of sites. However, variation in site characteristics and turbine arrangement and design at WRAs has made it difficult to derive conclusive patterns. Although some research indicates that end-of-row turbines are correlated with higher raptor fatalities because of their proximity to canyons and steep terrain, other studies have indicated that irregular spacing patterns and differences in turbine densities might be correlated with higher fatality risk.


Placing turbines in association with specific topographic features within WRAs may have an effect on the frequency of bird collisions. Turbines associated with canyons, mid-row depressions, and ridge ends have been correlated with higher fatalities. Models that identify risk in association with topographic features as well as spatial configurations and turbine design characteristics will be useful in determining turbine selection and their suitable placement at current high-risk WRAs. A model currently being developed at the Altamont Pass WRA and funded by the California Energy Commission’s Public Interest Energy Research–Environmental Area (PIEREA) could be used to identify, modify or remove existing high-risk turbines, and could also be applied during future projects to help site the location of newer replacement turbines.


Avoidance of high-risk areas where species susceptible to collision risk occur in relative abundance is the best solution for future wind energy developments. The National Wind Coordinating Committee (NWCC) has produced a handbook to address wind generation siting and permitting issues, and it recommends measures to avoid, potential high-risk areas, and the use of standardized study techniques. The NWCC also developed a comprehensive guide to standardized methods and metrics for determining potential risk and impacts to birds at existing and future wind farm sites.


At the Altamont Pass WRA, early research also raised concern about bird fatalities caused by electrocution and wire collisions. Retrofitting distribution lines with risk-reduction devices occurred throughout much of the Altamont Pass WRA and appears to have reduced bird electrocutions significantly. It appears that there is technology available to avoid most electrocution and wire collision risk at wind facilities and risk appears to have been reduced considerably at existing facilities. At new facilities, with the use of underground or raptor-safe transmission lines, installation of perch guards and the elimination of guyed wind turbines and meteorological towers, virtually all avian wire collision and electrocution fatality risk can be avoided.


Few bat fatalities have been reported at California WRAs, whereas a few facilities outside of California have reported a larger number. Some bat biologists have suggested the possibility that these fatalities could have a significant impact on bat populations. There are several special status bat species in California. The actual number of fatalities that now occur is likely underestimated, because bats are much more difficult to detect than most bird species during conventional bird fatality searches. Researchers cannot use the same survey methods or equipment to perform bat use surveys at wind facilities that they use to survey birds. Bat experts need to be consulted to identify potential bat threats at specific WRAs and to develop an appropriate carcass recovery methodology.


Future research should focus on the refinement of pre-project risk assessment and risk reduction at existing sites. The information gained to date, in conjunction with preconstruction assessments of bird use, has made it possible for some wind turbine developments to reduce some avian fatality risk significantly. However, more effective and economical risk-avoidance techniques need to be developed for proposed wind facilities. In addition, fatalities at the Altamont Pass WRA continue to be high, and new developments proposed at sites with known high bird use (such as Solano County) warrant further research to develop techniques to further reduce risk. Future research should concentrate on completing the analyses needed to understand the factors that contribute to risk, developing methods to reduce those risks, and monitoring to determine the effectiveness of those measures.

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