Birds and bats
Although birds do collide with wind turbines at some sites, modern wind power plants are collectively far less harmful to birds than numerous other hazards. The leading human-related causes of bird kills in the United States, according to the US Fish and Wildlife Service, are cats (1 billion deaths per year), buildings (up to 1 bn), hunters (100 million), vehicles (60 to 80 m), as well as communications towers, pesticides and power lines. Bird deaths due to wind development will never be more than a very small fraction of those caused by other commonly-accepted human activities, no matter how extensively wind is used in the future.
Avian studies carried out at many wind farm sites in the US show that bird kills per megawatt of installed capacity average one to six per year or less. These include sites passed by millions of migrating birds each year. At a few sites, no kills have been found at all. In Europe, studies of almost 1,000 wind turbines in the region of Navarra, Spain, showed a detected mortality rate of between 0.1 to 0.6 collisions per turbine per year.
Well publicised reports of bird deaths, especially birds of prey, at sites including the Altamont Pass near San Francisco and Tarifa in southern Spain, are not indicative of the day to day experience at the thousands of wind energy developments now operating around the world.
As a general rule, birds notice that new structures have arrived in their area, learn to avoid them, especially the turning blades, and are able to continue feeding and breeding in the location. Problems are most likely to occur when the site is either on a migration route, with large flocks of birds passing through the area, or is particularly attractive as a feeding or breeding ground. This can be avoided by careful siting. Modern wind turbines, with their slower turning blades, have proved less problematic than earlier models.
Bird studies are routinely carried out at prospective wind sites in order to understand the local pattern of breeding and feeding. Pre-construction wildlife surveys by a professional consultant are common practice. These surveys help reduce the threat to birds to a minimal level.
Like birds, bats are endangered by many human activities, from pesticide poisoning to collision with structures to loss of habitat. Despite publicity given to bat deaths around wind farms, mainly in the United States, studies have shown that wind turbines do not pose a significant threat to bat populations. A review of available evidence by ecological consultants WEST concluded that “bat collision mortality during the breeding season is virtually non-existent, despite the fact that relatively large numbers of bat species have been documented in close proximity to wind plants. These data suggest that wind plants do not currently impact resident breeding populations where they have been studied.”
Monitoring of wind farms in the US indicates that most deaths involve bats that are migrating in late summer and autumn. The American Wind Energy Association has now joined forces with Bat Conservation International, the US Fish and Wildlife Service and the National Renewable Energy Laboratory to look at why these collisions occur and how they can be prevented. This initiative is focused on finding good site screening tools and testing mitigation measures, including ultrasonic deterrent devices to warn bats away from turbines.