Bird Mortality in Two Dutch Wind Farms: Effects of Location, Spatial Design and Interactions with Powerlines

Numerous field studies have assessed bird mortality rates in wind farms. However, results from different studies are often hard to compare due to differences in methodology. This makes it very difficult to draw conclusions and to use the results in the planning phase of new wind farms (e.g. how to mitigate impacts). In this study, it was attempted to assess how bird mortality rates are affected by (1) the location of the wind farm, (2) the spatial layout of the turbines, (3) the surrounding terrain and (4) the presence of other obstacles such as powerlines. This study involved the monitoring of 91 turbines in two contrasting wind farms in the Netherlands for five years. It used the same standardized search methodology, including experimental trials for carcass removal and search efficiency. The sites differ in location (coastal vs. inland), spatial layout, turbine dimensions, land use, bird community and flight intensity of birds. In addition, at one site powerlines were constructed halfway through the monitoring program. Any fatalities from these powerlines were also monitored in a separate monitoring program. This enabled a comparison of any differences in mortality rates or species composition between the turbine and powerline fatalities. The results show a major impact of turbine location on the number of bird fatalities, both within the same wind farm and between wind farms. Mortality rates at the coastal wind farm were three to five times higher than at the inland wind farm. By far the highest mortality rates were found at turbines close to high-tide roosts and at points where (during spring migration) migrating birds leave the coastline to cross the sea towards Germany or Scandinavia. At these turbines, mortality rates could rise up to several hundred of birds per turbine per year. When expressed in fatalities per ha, overall fatality rates of the powerlines were three times higher than of the turbines in the same area. This may be due to low visibility of the powerlines compared to wind turbines. Comparison of turbine versus powerline fatalities also showed major differences in species composition, with powerline fatalities mostly consisting of passerines and waterfowl, and turbine fatalities being dominated by gulls. As several new wind farms are planned to be realized in the coming years, the results of this study can be used in spatial planning to both assess and mitigate potential impacts.