Bird collisions at wind turbines in a mountainous area related to bird movement intensities measured by radar

Bird collisions at wind turbines are perceived to be an important conservation issue. To determine mitigation actions such as temporary shutdown of wind turbines when bird movement intensities are high, knowledge of the relationship between the number of birds crossing an area and the number of collisions is essential. Our aim was to combine radar data on bird movement intensities with collision data from a systematic carcass search. We used a dedicated bird radar, located near a wind farm in a mountainous area, to continuously record bird movement intensities from February to mid-November 2015. In addition, we searched the ground below three wind turbines (Enercon E-82) for carcasses on 85 dates and considered three established correction factors to extrapolate the number of collisions. The extrapolated number of collisions was 20.7 birds/wind turbine (CI-95%: 14.3-29.6) for 8.5 months. Nocturnally migrating passerines, especially kinglets (Regulus sp.), represented 55% of the fatalities. 2.1% of the birds theoretically exposed to a collision (measured by radar at the height of the wind turbines) were effectively colliding. Collisions mainly occurred during migration and affected primarily nocturnal migrants. It was not possible to assign the fatalities doubtlessly to events with strong migration. Fresh-looking carcasses were found after nights with both strong and weak bird movement intensities, indicating fatalities are not restricted to mass movement events (onshore). Rather, it is likely that an important factor influencing collision risk is limited visibility due to weather conditions. Local and regional visibility should be considered in future studies and when fine-tuning shutdown systems for wind turbines.