Wind turbines in high quality habitat cause disproportionate increases in collision mortality of the white-tailed eagle

The increasing number of wind farms for energy production raises concerns about their effects on wildlife and particularly on birds. To date it is unclear whether models that combine data on wind turbine densities and habitat suitability can explain the actual spatial occurrence of collision fatalities and how well these models perform in comparison to models including measures of bird population densities (e.g., the distribution and density of nest sites). Here we analysed whether collision mortality increases with wind turbine density and whether a high population density or habitat suitability in the vicinity of wind turbines amplifies the effect of wind turbine density on collision mortality. We combined opportunistic records of dead White-tailed Eagles by the public in Northeast Germany during the period 2003 to 2014 with data on the distribution of wind turbines, nest sites and habitat suitability. As expected, wind turbine density was a strong predictor of collision mortality. In addition, we found that wind turbine density and habitat suitability had synergistic effects on collision mortality, so that the effect of wind turbine density was amplified in areas of high habitat suitability. Moreover, combining wind turbine density and habitat suitability allowed for better predictions of collision mortality than combining wind turbine density and nest site density. These results suggest that assessments of the spatial occurrence of collision fatalities based on models that combine data on wind turbine densities and habitat suitability can be useful for the strategic planning of wind farm development on regional scales. In particular, our study highlights that wind turbines should not be placed in core population areas of vulnerable bird species because synergies between wind turbine densities and habitat suitability may cause disproportionate increases in mortality. This might undermine the positive effects of parallel conservation efforts.