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ABSTRACT. In recent years, the eastern foothills of the Rocky Mountains in northeastern British Columbia have received interest as a site of industrial wind energy development but, simultaneously, have been the subject of concern about wind development coinciding with a known migratory corridor of Golden Eagles (Aquila chrysaetos). We tracked and quantified eagle flights that crossed or followed ridgelines slated for one such wind development. We found that hourly passage rates during fall migration peaked at midday and increased by 17% with each 1 km/h increase in wind speed and by 11% with each 1°C increase in temperature. The propensity to cross the ridge tops where turbines would be situated differed between age classes, with juvenile eagles almost twice as likely to traverse the ridge-top area as adults or subadults. During fall migration, Golden Eagles were more likely to cross ridges at turbine heights (risk zone, < 150 m above ground) under headwinds or tailwinds, but this likelihood decreased with increasing temperature. Conversely, during spring migration, eagles were more likely to move within the ridge-top area under eastern crosswinds. Identifying Golden Eagle flight routes and altitudes with respect to major weather systems and local topography in the Rockies may help identify scenarios in which the potential for collisions is greatest at this and other installations.RÉSUMÉ. Récemment, les contreforts des Rocheuses dans le nord-est de la Colombie-Britannique ont attiré l'attention comme site potentiel d'un développement éolien industriel, tout en faisant simultanément l'objet de préoccupations puisque ce projet coïnciderait avec un corridor de migration connu d'Aigles royaux (Aquila chrysaetos). Nous avons suivi et quantifié les vols d'aigles qui ont traversé ou longé les lignes de crêtes visées par un projet éolien de ce genre. Nous avons constaté que le taux de passage horaire durant la migration automnale atteignait un maximum à midi et augmentait de 17 % pour chaque km/h d'augmentation de la vitesse du vent, et de 11 % pour chaque °C d'augmentation de la température. La propension à traverser les sommets des crêtes où seraient installées les éoliennes différait selon les classes d'âge, les jeunes aigles de l'année ayant deux fois plus de chance de le faire que les adultes ou les jeunes plus âgés. Durant la migration automnale, les aigles traversaient davantage les crêtes à hauteur d'éoliennes (zone de risque, < 150 m au-dessus du niveau du sol) sous un vent de face ou arrière, mais cette tendance diminuait avec l'augmentation de la température. En revanche, durant la migration printanière, les aigles étaient plus susceptibles de survoler la région des sommets sous un vent latéral de l'est. La détermination des trajectoires et des altitudes de vol des Aigles royaux, selon les systèmes météorologiques prédominants et la topographie locale des Rocheuses, peut contribuer à identifier les scénarios dans lesquels les risques de collision sont les plus élevés, que ce soit pour ce projet éolien ou d'au...

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.

High resolution numerical atmospheric modeling around a mountain ridge in Northeastern British Columbia (BC), Canada was performed in order to examine the influence of meteorology and topography on Golden Eagle migration pathways at the meso-scale (tens of km). During three eagle fall migration periods (2007-2009), local meteorological conditions on the day of peak bird counts were modeled using the Regional Atmospheric Modeling System (RAMS) mesoscale model. Hourly local surface wind speed, wind direction, temperature, pressure and relative humidity were also monitored during these migration periods. Eagle migration flight paths were observed from the ground and converted to three-dimensional tracks using ArcGIS. The observed eagle migration flight paths were compared with the modeled vertical velocity wind fields. Flight tracks across the study area were also simulated using the modeled vertical velocity field in a migration model based on a fluid-flow analogy. It was found that both the large-scale weather conditions and the horizontal wind fields across the study area were broadly similar on each of the modeled migration days. Nonetheless, the location and density of flight tracks across the domain varied between days, with the 2007 event producing more tracks to the southwest of the observation location than the other 2 days. The modeled wind fields suggest that it is not possible for the eagles to traverse the study area without leaving updraft regions, but birds do converge on the locations of updrafts as they move through the area. Statistical associations between observed eagles positions and the vertical velocity field suggest that to the northwest (and to a lesser extent the southwest) of the main study ridge (Johnson col), eagles can always find updrafts but that they must pass through downdraft regions in the NE and SE as they make their way across the study area. Finally, the simulated flight tracks based on the fluid-flow model and the vertical velocity fields are in general agreement with the observed flight track patterns. Our results suggest that use of high resolution meteorological fields to locate the occurrence of updrafts in proposed ridge-line wind installations could aid in predicting, and mitigating for, convergence points in raptor migrations.

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