Abstract
Potential wind-energy development in the eastern Rocky Mountain foothills of British Columbia, Canada, raises concerns due to its overlap with a golden eagle (Aquila chrysaetos) migration corridor. The Dokie 1 Wind Energy Project is the first development in this area and stands as a model for other projects in the area because of regional consistency in topographic orientation and weather patterns. We visually tracked golden eagles over three fall migration seasons (2009–2011), one pre- and two post-construction, to document eagle flight behaviour in relation to a ridge-top wind energy development. We estimated three-dimensional positions of eagles in space as they migrated through our study site. Flight tracks were then incorporated into GIS to ascertain flight altitudes for eagles that flew over the ridge-top area (or turbine string). Individual flight paths were designated to a category of collision-risk based on flight altitude (e.g. flights within rotor-swept height; ≤150 m above ground) and wind speed (winds sufficient for the spinning of turbines; >6.8 km/h at ground level). Eagles were less likely to fly over the ridge-top area within rotor-swept height (risk zone) as wind speed increased, but were more likely to make such crosses under headwinds and tailwinds compared to western crosswinds. Most importantly, we observed a smaller proportion of flights within the risk zone at wind speeds sufficient for the spinning of turbines (higher-risk flights) during post-construction compared to pre-construction, suggesting that eagles showed detection and avoidance of turbines during migration.
Highlights
The construction of wind-energy installations in areas regularly used by raptors raises concerns over potential collision fatalities [1,2]
We found the likelihood for a golden eagle to cross over the ridge-top area within the risk zone (#150 m ag) increased by: 6 times during pre- versus post-construction years; 2.5 times under headwinds; and 8 times under tailwinds vs western crosswinds (LR-test x12 = 60.60, P#0.001; cross-validation estimate of predictable accuracy = 87%; Table 2)
The probability to cross over the ridge-top area at turbine height did not differ between golden eagle age categories, ridge-top topography features or between the 1.5 km long control area compared to the 3 km area with turbines
Summary
The construction of wind-energy installations in areas regularly used by raptors raises concerns over potential collision fatalities [1,2]. In Spain, resident Griffon Vulture (Gyps fulvus) collisions with turbines increased under weak-wind conditions along gentle slopes. Under such non-lift generating wind conditions, birds were forced to gain altitude by using slow circle-soaring flight on thermals, often in airspace that overlapped with turbines [4]. Some topography features of ridgelines generate vertically-deflected air (orographic lift) that provide sources of lift to soaring birds – depending upon the altitude of birds traversing such features, wind development on these same ridges could be associated with higher collision potential for raptors [3,4,16,17,18,19]
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