Age and season-related habitat selection patterns of the bearded vulture (Gypaetus barbatus) in the Swiss Alps: a basis for predicting conflict-zones with wind energy construction

Abstract

The recent increase of wind energy use in Central Europe incurs potential impacts on wildlife. Large soaring raptors, like the bearded vulture, are particularly exposed to collision risk with wind turbines as they sometimes converge in selecting fairly similar combinations of landscape and wind conditions. Considerable efforts and resources have been invested to re-instate the species in the European Alps. There exists a risk, however, that this success will be jeopardized by the sprawl of the wind parks across the alpine massif. We used a maximum entropy modelling approach to predict the potential distribution of the bearded vulture across the Swiss Alpine range using presence-only data. We adopted a stepwise fashion to tune model complexity by varying feature combinations and regularization intensity, selecting the settings that provided the most parsimonious model. We identified and ranked the environmental variables most relevant for the species and tested for differences in ecological requirements between two different age classes (adults and juveniles) in both the cold and the warm season separately. The resulting models had a high accuracy in predicting habitat suitability (mean AUC across 5-folds cross validation ≥ 0.81) in each season for both age classes. Adults and juveniles showed different seasonal habitat selection patterns: whereas for juveniles the most important environmental variable was food availability, particularly ibex density (relative contribution: 40.9% in summer and 25.9% in winter), for adults climatic conditions were more important (altitude with 24.9% contribution in summer and average precipitation with 30.6% contribution in winter). When considering both age classes 67% of the Swiss Alpine range offered suitable habitat for the species, with range shifts between the cold and warm season. This analysis provides a first, broad-scale overview of the species distribution across the Swiss Alps and thus areas of potential conflict with wind energy construction. We will now further investigate flight altitudes and movement patterns at a fine spatial scale in order to identify the sites bearded vultures use most intensively at risky flight heights, i.e. within the rotor-swept area. The results will be integrated into a planning tool that will help avoiding conflicts between wind energy construction and vulture conservation.