|Fig. 1: Bird flying toward an active wind turbine. (Source: Wikimedia Commons)|
Wind energy is a promising alternative to fossil fuels that produces electricity by capturing a portion of the available power in wind. This is accomplished via one or many wind turbine(s) which make up a wind farm. Wind energy has gained popularity over the years due to its ability to produce power with significantly fewer environmental pollutants relative to fossil fuels and other conventional energy sources.  Wind energy has experienced an average annual growth of 124% from 1990 to 2014. In the year 2013, 2.5% of global electricity was produced via wind and is expected to grow upwards of 15% by 2050.  Wind energy may seem objectively better for the planet due to its lack of obvious harmful environmental effects, especially when compared to fossil fuels. However, upon closer examination, wind farms have been noted to exhibit environmental and wildlife-related drawbacks including unnatural bird deaths, among others. 
Wind farms can harm wildlife populations in various ways. Birds and bats are noted to be particularly susceptible to habitat disruption, disturbance, and death by collision, all due to wind farms.  Fig. 1 shows the beginning of a potential collision as a bird flies between the blades of a medium-sized wind turbine. The combined effect of collision, disturbance, and disruption are more pronounced in other chief means of power generation than for wind farms.  Even so, the amassed impacts of new wind farms may significantly affect various more-vulnerable populations.  A single wind turbine can potentially kill up to 40 birds per year.  Using this statistic alone is insufficient to take action because it varies hugely across different wind farms along with the specific species killed. The high death rates of birds observed at various wind farms has triggered concern among the public and relevant parties. A good example to note is the high fatality rate of Golden eagles at California's Altamont Pass Wind Resource Area.  Hence, many studies have been employed to address the issue. 
Currently, there is no global agreement regarding the primary cause or method of mitigation for bird deaths as involved factors are extremely diverse.  Causes range from species-specific factors such as their body size and shape, behavior, sensory perception, and abundance, to wind farm-specific factors including landscape features, flight paths, weather, food, lights, and wind farm configurations.  Many ways to mitigate bird deaths have been suggested. The method believed to be the most effective is intentionally placing wind farms at sites where bird presence is minimized and out of migratory bird routes.  Such a site is determined using extensive modeling, the likes of which are still in development.  Other options include turning turbines off (stopping the spinning blades) during specific times, such as a foggy day, and painting the turbines to make them more visible to nearby birds. Those mentioned above are just a few examples out of the many methods that have been considered. No one method is straight-forward or universally effective. There is still much room for study, collaboration, and improvement. 
An argument could be posed that wind farms are actually beneficial to avian populations despite the information discussed above. In isolation, wind turbines kill a large number of birds and bats. However, now we must bring this discussion to encompass the effect of all energy generation and how it generally affects avian species. According to a self-declared "rudimentary" study presented in  intended to spark future rigorous studies, wind farms, nuclear power stations, and fossil fuel generators are each responsible for 0.28, 0.41, and 0.17 bird deaths per gigawatt-hour (GWh) of energy produced, respectively. This data does not include power transmission or climate change effects. The author of  asserts that climate change effects from fossil fuels "can be quantified into 4.98 deaths per GWh", bringing bird morality to 5.15 deaths per GWh.  Although the study discussed in  is merely preliminary, it demonstrates the need to contextualize wind energy-related bird deaths with other methods of energy generation. While wind, nuclear, and fossil fuels all share disruption, disturbance, and collision as factors that harm avian populations, nuclear and fossil fuels have other externalities to consider. Examples include acid rain and mercury emission for fossil fuel plants and uranium contamination for nuclear plants.  The very nature of electricity generation is hazardous for wildlife to some degree. Such negative effects must be analyzed across space and time to gain an accurate understanding for the true extent of the damage.
© Aaron Scherr. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
 A. T. Marques et al., "Understanding Bird Collisions at Wind Farms: An Updated Review on the Causes and Possible Mitigation Strategies," Biol. Conserv. 179, 40 (2014).
 T. Laranjeiro, R. May, and F. Verones, "Impacts of Onshore Wind Energy Production on Birds and Bats: Recommendations for Future Life Cycle Impact Assessment Developments," Int. J. Life Cycle Ass. 23, 2007 (2018).
 B. K. Sovacool, "The Avian Benefits of Wind Energy: A 2009 Update," Renew. Energy 49, 19 (2013).