Fatalities of birds and bats have been documented at wind power developments around the world. Particular attention has been given to deaths of raptors (hawks and eagles) in the United States because of a documented high rate of collisions of birds with wind turbines at the Altamont Pass Wind Resource Area in central California. State and federal laws prohibit unauthorized killing of raptors and most other birds, along with bats. Even a small number of deaths could have significant impacts on local bird and bat populations.
This study was conducted to ascertain actual and potential impacts on populations of birds and bats at the National Wind Technology Center (NWTC) in northern Jefferson County, Colorado. The NWTC, which is part of the U.S. Department of Energy’s National Renewable Energy Laboratory (NREL), is located on a mesa dominated by ungrazed grassland with isolated patches of ponderosa pine. Similar lands to the north and west are part of the city of Boulder’s “open space” system. Areas to the east and south are part of the Rocky Flats Environmental Technology Site.
NREL specified these core questions to be addressed in the study:
- What levels of bird and bat mortality are associated with the present NWTC facility, and how might this change with future expansions at the site, and with regional land use changes?
- How does the present NWTC facility affect abundance and movement patterns of birds and bats, both on the site and regionally?
- How might these abundance and movement patterns change in the future as the NWTC site is expanded, and as regional land use patterns change?
Field methods included fixed-distance point counts of both raptor and non-raptor species, and visual, acoustic, and capture surveys of bats, using plots both on and near the NWTC site. Behavioral responses of birds to wind turbines and other structures at the NWTC were recorded, as were general movement patterns of raptors. Finally, carcass searches were conducted and calibrated by tests of scavenging rates and searcher efficiencies to compare bird and bat mortality on and off the site. In this report, studies of diversity, status, and mortality of birds (Part I) and bats (Part II) are reported separately because of the differences in the biology of the study organisms, the techniques needed to study them, and the field personnel responsible.
Salient findings of the study were as follows:
- Abundances of individual raptor species on the NWTC site were similar to surrounding areas. However, the average number of species detected per count at the NWTC was nearly double that of surrounding areas in winter, the season when raptors are most abundant in the region. This difference is likely attributable to increased availability of perches at the site. Raptors flew and perched higher at the NWTC than in adjacent areas, again probably related to the wind turbines and other structures at the site.
- Only 1 of 46 bird species counted on grassland plots during this study differed in abundance between the NWTC and adjacent areas—the horned lark, which was about 16 times more common off site. This difference is attributable to cattle on Boulder Open Space creating low-stature grasslands preferred by this species.
- Bird abundance and variety on the site south of NWTC slated for future use were generally similar to the developed areas, except for the relative scarcity of raptors on the undeveloped site, which probably was due to a lack of perches.
- The NWTC does not support a large diversity or abundance of bat species (possibly six species of bats use the site), but an area on the northwest side of the site, with trees close to a rocky outcrop, provides foraging and perhaps roosting habitat.
- We found no raptor carcasses during our 12-month survey of the NWTC, except one American kestrel that had died before the study started. Bird mortality associated with the site appears to be minor. Approximate annual bird mortality attributable to the NWTC was 24 individuals, all songbirds (Passeriformes). Most of these deaths were probably the result of collisions with support wires for the meteorological towers rather than the turbines themselves. We found no evidence of bat fatalities at the site.