Research Paper: Destruction of Birds by Windmills
Birds are likely to be affected and/or destroyed by windmills, this is a natural part of life. The basic cycle is, wind developments tend to be placed in upland areas with strong wind currents that have a lot of potential to generate energy. Birds use these currents as highways, so they are likely to come into contact with the windmills (aka turbines). Windmills pose one the highest risks in the world of killing birds. Research also indicates that wind developments can disrupt migration routes; while bird's foraging and nesting habitat are at risk of being lost when turbines are put up.
Windmills don't affect all birds, but they do affect birds of prey, disproportionately. The main reason this occurs has to do with bird's genetics. For example, species like vultures have blind spots in their visual field. This means they cannot see objects directly in front of them (like wind turbines) when flying. Concerning amounts of bird deaths; large birds like hen harriers, eagles and vultures are slower to reproduce than other species, so their populations are more likely to have a small number of deaths. Unfortunately, there are specific locations in the world where windmills have caused a great number of fatalities amongst birds of prey. In the Almont pass in California, one study found that about 4,000 wind turbines killed 67 golden eagles and 1,127 birds of prey in a year. In Southern Spain, 252 wind turbines located in an area used by many birds of prey (on the migratory path of many large birds) killed 124 birds of prey in a year. At another location in southern Spain, 256 turbines killed 30 griffin vultures and 12 common kestrels.

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The RSPB (Royal Society for the Protection of Birds) references these studies, and concluded that some poorly sited wind mills in California and Spain have caused the most