Behavioural and environmental correlates of soaring-bird mortality at on-shore wind turbines

1. Wind power plants represent a risk of bird mortality, but the effects are still poorly quantified. We measured bird mortality, analysed the factors that led birds to fly close to turbines, and proposed mitigation measures at two wind farms installed in the Straits of Gibraltar, one of the most important migration bottlenecks between Europe and Africa. 2. Bird corpses were surveyed along turbine lines and an associated power line to estimate mortality rates. The behaviour of birds observed within 250 m of turbines was also recorded as a putative indicator of risk. The effects of location, weather and flight behaviour on risk situations (passes within 5 m of turbines) were analysed using generalized linear modelling (GLM). 3. Mortality caused by turbines was higher than that caused by the power line. Losses involved mainly resident species, mostly griffon vultures Gyps fulvus (0.15 individuals turbine(-1) year(-1)) and common kestrels Falco tinnunculus (0.19 individuals turbine(-1) year(-1)). Mortalities were not associated with either structural attributes of wind farms or visibility. 4. Vulture collisions occurred in autumn-winter and were aggregated at two turbine lines where risks of collisions were greatest. The absence of thermals in winter forced vultures to use slopes for lift, the most likely mechanism influencing both their exposure to turbines and mortality. 5. Kestrel deaths occurred during the annual peak of abundance in summer. Carcasses were concentrated in the open habitats around a single wind farm and risk may have resulted from hunting habitat preferences. 6. Synthesis and applications. We conclude that bird vulnerability and mortality at wind power facilities reflect a combination of site-specific (wind-relief interaction), species-specific and seasonal factors. Despite the large number of migrating birds in the study area, most follow routes that are displaced from the facilities. Consequently, only a small fraction of birds on migratory flights was actually exposed to turbines. New wind installations must be preceded by detailed behavioural observation of soaring birds as well as careful mapping of migration routes.