This article summarizes the threats that wind farms pose to birds before surveying the recent literature on avian mortality and summarizing common methodological problems with such studies. Based on operating performance in the United States and Europe, the paper then offers a preliminary calculation of the number of birds killed per kilowatt-hour kWh generated for wind electricity, fossil fuel, and nuclear power systems. The study estimates that wind farms and nuclear power stations are responsible each for between 0.3 and 0.4 fatalities per gigawatt-hour (GWh) of electricity while fossil fueled power stations are responsible for about 5.2 fatalities per GWh. Within the uncertainties of the data used, the estimate means that wind farms killed approximately 20,000 birds in the United States in 2009 but nuclear plants killed about 330,000 and fossil fueled power plants more than 14 million. The paper concludes that further study is needed, but also that fossil fueled power stations appear to pose a much greater threat to birds and avian wildlife than wind farms and nuclear power plants. (C) 2012 Published by Elsevier Ltd.