Bird Deaths at Wind Farms (Part 4)

Smithsonian-Sponsored Study of Bird Deaths at Monopole Wind Farms 

In a study published in December 2013, three scientists have attempted to estimate the number of bird deaths associated with tower design and height. See Scott R. Loss, Tom Will, Peter P. Marra, Estimates Of Bird Collision Mortality At Wind Facilities In The Contiguous United States. 

The sponsors of the study included the Migratory Bird Center, Smithsonian Conservation Biology Institute, National Zoological Park; the U.S. Fish and Wildlife Service, Division of Migratory Birds, Midwest Regional Office; and the Oklahoma State University

The study suggests that the trend toward taller towers could be leading to more deaths. On the other hand, the new research also reveals that siting fewer new turbines in California and more in the Great Plains – where the wind resource is rich and increasingly being exploited – could decrease the risk to birds.

This study did not focus at the species level, but instead analyzed available data to model bird mortality in an era in which lattice towers have largely given way to monopole towers with turbine hub heights often 80 meters or higher, and with longer turbine blades.

The researchers estimated that between 140,000 and 328,000 birds are killed annually by the monopole turbines, “which comprise the vast majority of all installed U.S. wind turbines.” Their median of 234,000 is thus a good deal lower than the Smallwood study earlier in 2013 that put the figure at 573,000.   

But in one of a barrage of caveats contained in the study, the authors said their assumption that all turbines are monopole could affect their total predicted fatalities.

Finally, the authors predict annual mortality estimate of roughly 1.4 million birds if the U.S. reaches its goal of relying on wind energy for 20 percent of its electricity supply.

Abstract of Smithsonian Study:

Wind energy has emerged as a promising alternative to fossil fuels, yet the impacts of wind facilities on wildlife remain unclear. Prior studies estimate between 10,000 and 573,000 fatal bird collisions with U.S. wind turbines annually; however, these studies do not differentiate between turbines with a monopole tower and those with a lattice tower, the former of which now comprise the vast majority of all U.S. wind turbines and the latter of which are largely being de-commissioned.

We systematically derived an estimate of bird mortality for U.S. monopole turbines by applying inclusion criteria to compiled studies, identifying correlates of mortality, and utilizing a predictive model to estimate mortality along with uncertainty. Despite measures taken to increase analytical rigor, the studies we used may provide a non-random representation of all data. Requiring industry reports to be made publicly available would improve understanding of wind energy impacts. Nonetheless, we estimate that between 140,000 and 328,000 (mean = 234,000) birds are killed annually by collisions with monopole turbines in the contiguous U.S.

We found support for an increase in mortality with increasing turbine hub height and support for differing mortality rates among regions, with per turbine mortality lowest in the Great Plains. Evaluation of risks to birds is warranted prior to continuing a widespread shift to taller wind turbines. Regional patterns of collision risk, while not obviating the need for species-specific and local-scale assessments, may inform broad-scale decisions about wind facility siting.

Smithsonian Study Findings:

After accounting for varying proportions of the year being sampled, annual per turbine mortality was modeled to be highest in the East (median = 8.16 birds), followed by California (median = 4.82 birds), the West excluding California (median = 3.64 birds), and the Great Plains (median = 2.43 birds). The researchers estimate that 46.4% of total mortality at monopole wind turbines occurs in California, 23.1% occurs in the Great Plains, 18.8% occurs in the East, and 11.6% occurs in the West ( Table 2).

On a per MW basis, California had a mean collision rate of 18.76 birds per MW (95% CI = 9.68–27.84), followed by the East (3.86 birds/MW; 95% CI = 3.05–4.68), the West (2.83 birds/MW; 95% CI = 2.05–3.62), and the Great Plains (1.81 birds/MW; 1.00–2.62). Regional differences based on the additive region-height model are different from those based on the univariate region model because the latter were calculated independently of turbine height data.

More interesting, perhaps, is the tentative link the researchers draw between turbine height and avian risk. The researchers said their data set had hub heights ranging from 36 to 80 meters and as height increased, “annual model-predicted mortality increased nearly ten-fold (from 0.64 t 6.20 birds per turbine).”