Retrofitting of power lines effectively reduces mortality by electrocution in large birds: an example with the endangered Bonelli's eagle

Summary 1. Mortality caused by power lines is a conservation problem for many vulnerable bird species. Many large species are especially threatened by electrocution as they frequently perch on pylons leading to electrocution that typically causes death. Electrocution mitigation measures have been implemented to protect several species; however, a resulting decrease in mortality due to these measures has not previously been demonstrated at the population scale. 2. In this study, we used data from a long-term capture–recapture programme (combining resightings of live birds and recovery of dead birds) carried out on the French population of the Bonelli’s eagle Aquila fasciata from 1990 to 2009 to estimate the impact of the insulation of power lines on key demographic rates. 3. We found that the survival probability of all age classes increased after the insulation of dangerous power lines, due to a decrease in mortality caused by electrocution. This decrease was partially compensated for by an increase in other causes of death. 4. Our findings show that insulation of power lines has a strong positive impact on juveniles and immature birds and a lesser impact on adults. The overall increase in survival due to power line insulation led to a sharp increase in predicted population growth rates (from 082 to 098), although our findings still suggest that the population is not self-sustaining. Elasticity values indicate that adult survival is the key parameter in the population dynamics of this species, and since adult mortality caused by electrocution seemed close to zero, our ability to act on this parameter is limited. 5. This study demonstrates that insulation of power lines is relevant for the conservation of large bird species at a population scale as it allows the survival rate of all age classes to increase and thus in turn has a strong positive impact on population growth rates. 6. Synthesis and applications. We demonstrated that mortality rates induced by electrocution are considerable and have major consequences for the population viability of birds. We also demonstrated that electrocution mitigation measures can lead to a sharp increase in survival through reducing mortality from electrocution leading to improved population viability. In the light of these results, there is an urgent need that conservationists contact power line stakeholders not only to urge them to generalize retrofitting actions but also, in planning new infrastructure development, to plan for less harmful power lines, since this will be far less costly than developing a posteriori mitigation actions.