The National Renewable Energy Laboratory (NREL) contracted Tetra Tech EC, Inc. (Tetra Tech) to conduct avian monitoring and bird and bat mortality surveys at the National Wind Technology Center (NWTC) in 2010–2011 to quantify local avian use in the NWTC and vicinity and to assess potential impacts to avian and bat species associated with the wind turbines and associated meteorological (met) towers at NWTC. Several different types of wind turbines are present on the NWTC with different capacities for energy production (1 kilowatt–3 megawatts [MW]; varying hub heights and turbine blade lengths) and met towers (guyed and unguyed) that vary in height (9.1–135 meters [m]). Installation of three newer generation, multi-MW wind turbines (large turbines) has raised awareness regarding the potential for wildlife impacts. In a U.S. Department of Energy (DOE) review of the large turbine installations, DOE requested monitoring of avian and bat mortality be conducted, so that mitigation measures could be developed if impacts to avian and bat species are observed.
To quantify local avian use in the NWTC and vicinity, Tetra Tech conducted avian point count surveys at 6 locations within the NWTC, as well as at reference locations in the adjacent Rocky Flats National Wildlife Refuge (3 locations; Rocky Flats) and the nearby Boulder County Open Space (3 locations; Boulder County). Further, Tetra Tech conducted breeding bird transect surveys to assess use of the NWTC by breeding grassland birds. To assess potential impacts to avian and bat species associated with the operation of wind turbines and associated met towers at the NWTC, Tetra Tech conducted fatality monitoring surveys at all vertical structures within the NWTC including observer efficiency and carcass persistence trials to refine the mortality estimation. It is therefore important to note when considering the results that multiple factors influence the probability of a species to have negative interactions with turbines, met towers, or guy wires, including the number of individuals at the site and the frequency that the species flies at rotor-swept area (RSA) height. Additionally, the relationship between avian use and mortality is not yet completely well-defined (NWCC 2010). Conclusions discussed below were based upon available data.