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Keywords:

  • bovine spongiform encephalopathy;
  • Gyps fulvus;
  • mortality;
  • population model;
  • rescue effect;
  • scavenging birds;
  • wind farms

Summary

1. Wind farms are emerging as a major cause of mortality of large scavenging bird species, which may be catastrophic when they operate in concert with other threats. As a study model, we examine the impact of wind turbines on the population dynamics of a soaring bird species, when acting in conjunction with a sudden decrease in food availability following the European bovine spongiform encephalopathy (BSE) epidemic.

2. In Spain, vultures have been provided with supplementary food at traditional vulture restaurants for centuries. In 2006/2007, these feeding stations were closed as part of disease control measures. At the same time, wind farms were deployed within the vulture foraging range. We used capture–recapture data and direct observation to monitor the impacts of these changes on the vulture population.

3. The number of breeding pairs decreased by c. 24%, adult survival by 30% and fecundity by 35%. However, the population recovered as soon as the perturbations ceased, the vulture restaurants were reopened, and the most problematic wind turbines were closed. Population recovery was faster than predicted by a retrospective stochastic population model.

4. Our analyses indicate that fecundity and survival were influenced predominantly by wind turbines. Food scarcity promoted a shift in foraging behaviour that drove vultures to fly into the path of wind turbines as they sought out new food sources in a landfill site. Elasticity and sensitivity analyses of the population model showed that mortality of adult birds had a much greater effect on population declines than mortality of immature birds, whereas reduction in fecundity had negligible effects.

5. The most likely explanation for the rapid recovery of the vulture population is that the observed decline in breeding pairs was not solely because of increased mortality. The decline probably included dispersal away from the area and a greater incidence of skipped breeding during the perturbation years. Subsequent immigration from large nearby populations was probably a factor in population recovery.

6.Synthesis and applications. Where specific wind turbines are causing substantial mortality, their closure is an effective management response. For vulture populations dependent on supplemental feeding stations, the feeding sites should be relocated away from the most problematic wind turbines, or other anthropogenic sources of mortality, to prevent negative impacts. We recommend the establishment of scattered, low-value food sources to replicate historical conditions and to avoid the problems associated with high concentrations of individuals in one place.