The collisions of birds and bats with wind turbines has been noted since the 1970s, though only in light of the recent wind energy expansion has the problem been seriously recognised. Of the major studies recording bird strikes from wind turbines many quote collision rates per turbine from 0 to over 60 collision fatalities per year, which equals 0 to 20 birds per MW per year. Many bird species feature in the collision records, including gulls, raptors, such as griffon vulture, golden eagle, red kite, kestrels, and red-tailed hawks, though it is suggested that limited information existent on passerines collisions with wind turbines is probably due to a combination of fewer studies, lower detection rates, rapid scavenger removal.
Although the population level impacts are difficult to assess, studies of the Altamont Pass Wind Resource Area indicate further expansion is considered likely to lead to population decline unless adequate measures are taken to substantially reduce collision risk. Form the Tarifa and Smøla studies in is stressed that collision risk is highly site-specific and the Zeebrugge study highlights bird-specific behaviour as the cause. The important factors associated with elevated collision risk identified at onshore wind farms include topography, turbine location, design, and configuration, including spacing, and land use close to turbines, whereas off-shore wind farms, though difficult to assess, seem to have less of an impact.
Although the US predicted annual avian mortality as a result of collisions with wind turbines amount to <0.01% of anthropogenically-caused avian deaths, in certain areas wind farms do seem to be acting as population sinks. It is suggested that with the expanding wind industry the bird declines, initially restricted to local populations of the most vulnerable species, could eventually lead to regional or even national population declines. Rough estimates show that by 2020 the number of birds killed annually could range from 300-400 to 3,000-5,700, though could rise by 300,000 by 2030. By contrast, the adverse impacts of climate change projects 15% to 37% of all bird and wildlife species to be "committed to extinction" by 2050.
The siting, or location, of a wind farm seems to be the single most important factor contributing to the risk of bird fatalities, though within wind farm design and layout also have an impact. Predicting and assessing the siting impact is seen to be the most important management tool. Additional mitigation options include feathering the turbines during high-risk periods, making the blades more visible, reducing the lighting, reducing the attractiveness of the areas around the turbines, and/or bird deterrents.
Bat fatalities only really gained attention after 2003, when an estimated 1,400-4,000 bats were killed at the Mountaineer Wind Energy Centre in West Virginia at rates estimated above 30 per MW of installed capacity per year, which is well above the rates estimated for bird fatalities. Both US and European studies seem to suggest highly variable rates of mortality from 1 to 40 per MW per year in the US and from 1.5 to over 20 per turbine per year in the EU. Key findings from the studies reviewed include that most fatalities were dominated by lasiurine (migratory) species, with fatalities peaking in midsummer through autumn, though the specific local habitat influence has not yet been identified. It is also suggested that bat fatalities are highest during periods of low wind speeds, less than 6 m/sec, at which speeds wind turbines could be feathered as a potential mitigation option.
The peaking of bat fatalities during late summer and early autumn may be partially the result of exploratory activity, though additional factors, such as potential roost attraction, movement or sound attraction, or available prey, may help explain wind turbine-caused fatalities. Recent evidence suggests that bat fatalities is mainly caused by barotrauma, the rapid recompression experienced by bats due to changes in atmospheric pressure as the turbine blades rotate downward, in addition to direct rotor strike.
Although the data on bat fatalities at wind farms is still very sparse and sporadic, with, for example, no studies from Texas, which has the largest installed capacity of wind energy in the US, the population level impact is difficult to determine. Nonetheless, projected bat fatalities in the Mid-Atlantic Highlands could range from 33,000 to 110,000 annually by 2020. Furthermore, if the US supplies 20% of its electricity from wind by 2030, annual bat fatalities could be between 1,500,000 to 8,500,000 depending on the average number of deaths per MW installed.
In light of such potential impacts wind farm developers should take a number of impact mitigation steps, such as the five represented in the Californian Guidelines. These include preliminary site screening, permitting requirements and law compliance, a one-year pre-permitting assessment to determine the potential bird and bat impact of the site, impact analysis and mitigation, and finally operations monitoring for two years after the farm has been developed.