Nobody really knows how many birds are destroyed annually in wind turbine related collisions. Wind proponents have long discounted the carnage by pointing at other sources of bird mortality including cats, windows, and communications towers, but the issue still haunts developers.
This month, the American Wind Wildlife Institute (AWWI) released its latest analysis of bird fatalities with the focus on small passerines (commonly referred to as songbirds). Passerines are among the most frequently reported fatalities found at the base of the towers.
The report, sponsored by Big Wind and environmental groups, examined 116 post-construction bird mortality studies conducted at 70+ projects in North America between 1996 and 2012. AWWI concluded that turbines kill 229,765 passerines, and up to 368,000 of all bird species (including raptors), annually in the U.S. and Canada.
The problem for AWWI (and by implication the American Wind Energy Association), is that many researchers — and the public — aren’t buying the industry’s numbers, and for good reason.
Flawed Data, Flawed Studies
Determining bird mortality at a site involves the tedious work of searching around the towers looking for carcasses. Those that are found are counted, their species and cause of death catalogued, and notes are taken regarding the timing of the find, distance from the turbine and any evidence of scavenging. Facility-specific information is also collected including the number, height, and capacity of the turbines and the amount of vegetation in the area. Vegetation near the turbines reduces ground visibility and lessens the effectiveness of the search.
Intense debate persists regarding the adequacy of the searches and the accuracy of the final mortality estimates. Biologists unaffiliated with wind developers have called for standards but without much success. Significant variability in the methods that are followed, both in the field (searching/ counting carcasses) and modeling, raise doubt that continent-wide kill rates can be derived from the current data.
A full discussion of the flaws found in available mortality studies is beyond the scope of this essay, but there are several obvious problems with the field searches that shed light on why industry data cannot be trusted and could result in a significant under-reporting of bird mortality. All of the 116 mortality studies considered in AWWI's report suffer one or more of these flaws. (The full list of mortality studies reviewed by AWWI can be found here.)
Plot size: The size of the search area below a turbine must be large enough to account for most bird kills. US Fish and Wildlife Service (FWS) guidelines call for rectangular plots equal to twice the turbine height up to the blade tip. Others argue that more field research is needed to determine optimal search radii based on tower height, blade length and the distance carcasses can be flung. Plot sizes varied significantly across the 116 studies reviewed by AWWI with most being smaller than FWS guidelines and based on turbine hub height. Dead birds thrown outside the search boundaries were generally not counted.
Search intervals: Search intervals varied widely across all 116 studies. Many were conducted monthly or bi-monthly. Others adjusted the frequency of their searches depending on the time of year or when migration was occurring (ex: weekly searches in spring, fall; bi-monthly in summer, winter).
Still others maintained different search intervals at different turbines in the same project (ex: twice weekly for odd turbines; weekly for even turbines.) Longer search intervals bias the studies particularly if scavenging rates are high and not properly accounted for. Implementing different search intervals produces two or more kill rates at the same project raising questions about which is correct.
Number of years studied and seasonal timing: AWWI's report showed that the duration of the studies varied with many conducted over one year or less. One-year and partial year surveys are not adequate given the significant annual variations in bird activity that can occur from one year to the next. Also, studies have shown that concentrating searches in particular months or seasons can substantially underestimate mortality.
Geographic areas underrepresented: In looking at individual post-construction studies, it's apparent many geographic areas were poorly represented. Only three mortality studies applied to Texas projects, for example. Of those, two conducted searches at three-week intervals for less than a year.
Minnesota's surveys weren't much better. Of the ten mortality studies included for the state, seven dated back to 1996-1999 at the Buffalo Ridge wind plant (phases I, II and III) with a total wind capacity of just 236 megawatts, turbine hub heights at or below 50 meters, and partial year searches conducted bi-monthly.
All of Canada was represented by two studies, one in Ontario and one in Alberta, covering just 146 megawatts. This represents less than two percent of Canada’s 8,517 MW of installed wind capacity, which is centered in Ontario (2,856 MW) and Quebec (2,398 MW).
AWWI's True Intent
AWWI's report attempts to draw conclusions about continent-wide bird mortality based on very limited, highly biased data. It's evident from the literature that more needs to be done to understand bird (and bat) mortality, but the industry is becoming less, not more, cooperative. Transparency is the only way to inform the science and ensure safer siting practices yet, fatality reports are harder to find and most of the new studies (since 2010) are kept confidential.
But the true intent of AWWI's study is not about accurate mortality estimates. It's about deflecting the problem. In the final section of the paper, titled "Further Research" the authors arrogantly suggest we're wasting our time assessing turbine-related kills when we should be developing "solutions for reducing mortality from those other sources" - like cats and buildings.
The fact is, many more birds (and bats) are dying at operating wind plants than we know and as the turbines spread, mortality will increase. Now is not the time to relax our concern and look elsewhere. Rather, we should be doubling-down and demanding that the industry be held accountable for bird mortality once and for all!
 Bird fatalities caused by turbine collisions represent "direct effects." Few studies have examined the "indirect effects" of wind project siting where bird habitats are forever altered as a consequence of the project and related infrastructure including roads and transmission. Habitat fragment and nest destruction are creating significant stress birds.
 "High mortality figures associated with cats and windows predominantly involve plentiful species that are common in suburban and residential neighborhoods or in the vicinity of farms, whereas the species killed at commercial wind turbine facilities and communications towers are largely neotropical migrant songbirds; species of conservation concern that nest in our wild lands." William R. Evans, Critical review of Chautauqua Windpower, LLC Avian Risk Assessment.
Arnett et. al. (2008) Patterns of Bat Fatalities at Wind Energy Facilities in North America
Erickson WP, Johnson GD, Strickland MD, Young DP Jr, Sernka KJ, et al. (2001) Avian Collisions with Wind Turbines: A Summary of Existing Studies and Comparisons to Other Sources of Bird Collision Mortality in the United States.
J. Ryan Zimmerling et. al. (2013) Canadian Estimate of Bird Mortality Due to Collisions and Direct Habitat Loss Associated with Wind Turbine Developments
Kunz et.al. (2007) Assessing Impacts of Wind-Energy Development on Nocturnally Active Birds and Bats: A Guidance Document
Loss SR, Will T, Marra PP (2013) Estimates of Bird Collision Mortality at Wind Facilities in the Contiguous United States. Biol. Conserv. 168: 201–209.
Smallwood KS (2013) Comparing Bird and Bat Fatality-Rate Estimates Among North American Wind-Energy Projects. Wildlife Soc. B. 37: 19–33.