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 would save tens of million birds lives annually. In the USA, the best numbers show that roughly one in 86,000 birds are killed annually by wind energy. 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. Global warming and pollution are the threats; wind power is part of the solution, not a problem.
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.
Further, a recent assessment by the International Union for the Conservation of Nature (IUCN), the organisation that produces the Red List of Threatened Species makes it clear that up to 50% of bird species as well as many other categories of wildlife are at risk of extinction from global warming.
These are the big threats.
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.[9
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 234,000 birds (median number) 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.
A recent in depth study of avian mortality finds only 1.33 birds per turbine per year as the mean of deaths worldwide. 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 is also worth noting that this is at the low end of the scale of counts of wind turbine avian mortality, so should be taken as a point on the continuum as much as higher numbers are.
A very recent study was published in December of 2013 showed a reasonable mean of 234,000 birds killed by collisions with wind turbines annually in the USA, with bounds of 140,000 and 328,000.  Given the range of other studies from 30,000 to Smallwood’s high-end of 573,000 (calculated by taking 73 professionally executed empirical studies which had already been adjusted upward and adjusting them upwards further in one or more of four ways) , this seems like a much more likely number than both high-ball and lowball estimates. At those numbers, and looking at the estimated number of 20 billion birds in the USA after nesting, the median represents one in 86,000 birds in the USA killed annually by wind energy. With improvements in siting and mitigation, increasing wind generation to 30% of US demand would likely result in somewhere in the one in 10,000 birds killed by wind energy. Outside of endangered species, this is not significant.
Also worth noting is this recent peer-reviewed study from Canada, which concluded:
[T]hese values are likely much lower than those from collisions with some other anthropogenic sources such as windows, vehicles, or towers, or habitat loss due to many other forms of development. Species composition data suggest that < 0.2% of the population of any species is currently affected by mortality or displacement from wind turbine development. Therefore, population level impacts are unlikely, provided that highly sensitive or rare habitats, as well as concentration areas for species at risk, are avoided. 
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 and significant attention to improving siting. Attention to siting in larger bird migration routes is reasonable, as is attention to habitant for species in threat of extinction. This short documentary gives a good balanced view of ongoing efforts at Altamont Pass.
Raptors have been in the news recently as the US is considering allowing single digit takes of eagles by wind farms for periods of up to 30 years. This is reasonable as eagles are not endangered. There are some outliers and interesting points related to this as always. The Osage Tribe is fighting wind farms on their land and asserting that it’s their reverence for eagles for the basis for their fight, but they clearly state in their lawsuits and internal press that they are worried oil and gas extraction from their lands would be hindered by wind farm development. As eagles populations are much more threatened by global warming and pollution than wind farms, from the outside this can only be described as hypocritical. Further, evidence out of Australia where researchers spent two and a half years observing eagles at wind farms found that two different species very clearly avoided wind turbines. And of course, raptor populations are stable or rebounding at present in many cases.
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?
 Zimmerling, J. R., A. C. Pomeroy, M. V. d’Entremont, and C. M. Francis. 2013. Canadian estimate of bird mortality due to collisions and direct habitat loss associated with wind turbine developments. Avian Conservation and Ecology 8(2): 10. http://dx.doi.org/10.5751/ACE-00609-080210
 Estimates of bird collision mortality at wind facilities in the contiguous United States, Scott R. Loss, Tom Will, Peter P. Marra, Biological Conservation, Volume 168, December 2013, Pages 201–209, http://www.sciencedirect.com/science/article/pii/S0006320713003522
 Comparing bird and bat fatality-rate estimates among North American wind-energy projects, K. Shawn Smallwood, Wildlife Society Bulletin, Volume 37, Issue 1, pages 19–33, March 2013, DOI: 10.1002/wsb.260, http://onlinelibrary.wiley.com/doi/10.1002/wsb.260/abstract
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