| Summary: | We used two data-recording, open-array marine surveillance radars to track microscale movements of nocturnal migrants at a wind energy project in northeast British Columbia during the preoperational (2008-2010) and operational periods (2011-2012). Data was collected during the peak periods of spring and fall passerine migration in each year. We measured bearing and altitude of nocturnal migrants, as well as the average number of migrants flying in the airspace closest to the wind turbines. Using weather data on wind direction and strength during the periods of monitored migration, we calculated flow-assistance of wind in aiding migration. Although there was greater flow-assistance to movement in spring over fall migration, we did not find a significant difference between the preoperational and operational periods in flow-assisted flight. The altitude at which migrants flew did differ with development phase of the wind facility; migrants flew at higher altitudes during years when the turbines were operational compared to preoperational years. Although the proportion of migrants detected in the airspace 0-150 m above ground level (agl), coinciding with turbine height, did not differ with season or operational phase of the installation, there was a reduction in the proportion of migrants in the airspace just above turbines (151-300 m agl) when turbines were operational. In general, though, the overall altitudes used by migrants were typically higher than turbine height, so the adjustments we documented would only further reduce what appear to be already low levels of collision risk at this particular facility. We discuss possible reasons why this facility appeared to induce low collision risk to migrants, and how this might inform siting decisions of other wind facilities.
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