Wind turbines are often criticized for killing birds and bats. Fights against siting wind turbines in bird migration corridors or in bird habitat are frequent. Highly inflammatory language is used by anti-wind energy advocates such as ‘bird mincer’, ‘bird blender’ and ’eagle killers’. Outlandish numbers of deaths are often attributed to them.
How significant is the mortality of wind turbines upon birds and bats?
Replacing all fossil fuel generation with wind turbines world wide could save roughly 70 million birds lives annually. Bats are put at much more significant risk from fossil fuel and other human impacts than by wind turbines. Displacement of fossil fuel generation makes wind a strong net benefit to birds and bats.
Overall, wind energy has the least impact on wildlife of any form of energy generation with the possible exception of solar.
Every other form of generation has at some point in its lifecycle the possibility of:
- Large scale, population-level mortality and/or habitat destruction Population(s) decline and/or biodiversity is reduced
- A threat to species survival regionally
- Biologically significant mortality or reduction in endangered or threatenedspecies
- Limited, but locally to regionally important mortality and/or habitat destruction, with limited population-level effects
- Any biodiversity declines would be local to regional only
- No threat to species survival, but demonstrated effects to physiologyand/or behavior of exposed individuals
- Incidental mortality and/or incidental habitat destruction of endangered or threatened species
Wind energy at worst has only the possibility of:
- Limited and local mortality and/or habitat destruction, with no population- level effects
- Biodiversity declines are unlikely
- Endangered or threatened species may be exposed, but mortality unlikely
This is according to the most recent of two multi-energy source studies of wildlife mortality, Comparison Of Reported Effects And Risks To Vertebrate Wildlife From Six Electricity Generation Types In The New York/New England Region, prepared for the New York State Energy Research And Development Authority in 2009.
What about birds specifically?
Birds are killed as a result of human impacts in large numbers every year. The biggest human-related causes of deaths annually are, :
- Lighted window impacts – 97 to 976 million
- Predatory house cats – 500 million or more
- High-tension wire impacts – up to 174 million or more
- Pesticides – 72 million and possibly many more
- Car impacts – 60 million
Even these very large numbers are relatively small compared to the threat of habitat loss from acid rain from burning coal, open-pit mining for coal, mountain-top removal mining for coal and pollution.
These numbers are also very small compared to the 100-200 billion birds on the planet.  Adding all of the anthropogenic-impact causes of death above together might see 1.5 billion bird deaths, or 0.75% to 1.5% of the total. Unfortunate, but not species threatening except in very specific circumstances.
Wind turbines have been added to the list of bird killers in recent years. This is not because they kill significant numbers of birds; the worst cases has a handful of birds per turbine per year. According to the best impartial sources, they kill perhaps 30,000 – 60,000 birds annually in the USA. Of course, numbers of wind turbines are increasing, but so is siting sensitivity and mitigations (see below). Compared to the roughly 2 billion from other sources, this cannot be considered significant. Even doubling or tripling the number of mortalities still makes wind turbines a very small contributor to avian fatalities.
The most recent in depth study of avian mortality finds only 1.33 birds per turbine per year as the mean of deaths. This implies that for the 200,000 wind turbines world-wide, there would be roughly 266,000 birds killed. This is much smaller than critics state and still much smaller than deaths due to fossil fuel generation, but still something worth improving upon. Of specific note in this study is the lack of any correlation between predictions of avian mortality at wind farm sites and actual mortality.
It’s worth noting that while some wind farms kill a few birds per wind turbine per year, many wind farms kill almost no birds or actually no birds per year
However, there has been a noticeable absence or low frequency of avian deaths at other wind farms. Kerlinger (1997) conducted a five-month survey at the Searsburg, Vermont Wind Energy Facility and found no fatalities. Lubbers (1988) surveyed eighteen 300 kW wind turbines in Oosterbierum, Denmark, and found only 3 fatalities over 75 days, or less than 0.8 per turbine per year. Marsh (2007) found a bird casualty rate of 0.22 birds per turbine year after monitoring 964 turbines across 26 wind farms in Northern Spain. Rigorous observation of a 22-turbine wind farm in Wales documented that it has killed no birds, and researchers found a shift in bird activity to a neighboring area (Lowther, 1998). 
Wind turbines have tended to kill larger birds such as raptors and vultures in slightly higher numbers. This is important as there are generally fewer of the larger birds and in the case of raptors they are an apex predator. Threatening populations of these birds has been a concern, especially in the Altamont Pass, which is a raptor migration route. However, bird deaths per turbine have dropped off their in recent years with the elimination of older, lattice-tower turbines that were used as roosts by raptors. Attention to siting in larger bird migration routes is reasonable, as is attention to habitant for species in threat of extinction.
And a recent UK study on a 10 bird species near wind turbines found that construction disrupted populations slightly, but that operation did not cause any challenges for the majority of species, aided one species and only had a minor negative impact on numbers of one type of bird.
In general, song birds migrate at 2000-4000 feet, well above the level of wind turbines. Sea birds have been shown to avoid wind turbines based on radar and thermal imaging studies; one study found that millions of sea birds migrated past an offshore wind farm annually, and only two were killed.  This radar mapping of north and south migrations of migratory seabirds is very illustrative of what sea birds do:
It is also worth considering the alternative: more fossil fuel generation. 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. (Coincidentally, human fatalities per TWh of electricity are roughly 0.4 for nuclear and wind, and roughly 5 for coal according to one study; the very similar ratios between human and avian mortality are striking).
This graph from the energy policy report that established relative bird deaths for nuclear, wind and fossil fuel is telling: estimated world-wide avian mortalities in 2006.
According to Worldwide electricity production from renewable energy sources, 2011 Edition,  fossil fuels generated 14,264.4 TWh of electricity in 2010. Assuming this number and the ratio of mortality, replacing all fossil fuel generation with wind energy would save the lives of roughly 70 MILLION BIRDS ANNUALLY.
Mitigations for bird deaths continue to be pursued, including radar assessment of bird density causing wind farm feathering, painting the blades purple to avoid attracting birds during the day and avoiding steady white lights that attract insects and birds at night., 
What about bats?
Bats are also killed by wind turbines, once again a few bats per turbine per year in areas where bats are common, but rarely if ever through direct collision. University of Calgary studies show that bats are very able to avoid moving wind turbine blades through echo-location. However, they suffer from barotrauma — a significant pressure difference that disrupts their hearts and lungs — when they fly close behind the blades., 
Bat populations are not threatened by wind turbines. Bats are put at risk by many of the same things that put birds at risk: pesticides and habitat destruction among them.
White-nose syndrome is the biggest threat to bats right now in the USA. It has caused 300,000 deaths in one cave in the US alone, much more than the total bat deaths related to wind turbines., 
The industry and governmental agencies still take bat mortality due to wind turbines seriously. Where wind turbines impact sensitive populations, there have been interventions as significant as nightly shut downs of the entire wind turbine farm due to a single bat death. There is ongoing work to reduce bat deaths through radar, ultrasonic and higher-blade startup speeds., 
 Wind Energy: Do wind turbines have an impact on aquifers?
 Mike Barnard’s answer to Why are bats not better able to avoid wind turbines?
A subset of this material was published on RenewEconomy here: http://reneweconomy.com.au/2012/want-to-save-70-million-birds-a-year-build-more-wind-farms-18274