The Avian Risk Assessment (ARA) ... tries to make the case that mortality to night migrating birds will be insignificant based on the following evidence, listed in the order presented in the ARA's Executive Summary:
1. That wind turbine mortality is minor compared with other sources of mortality.
2. That their field data from the proposed project site along with mortality and field data from other wind projects indicate that the mortality from their proposed project will be minimal and in the range of other wind projects that have been studied.
The first point is standard rhetoric used in the commercial wind industry to minimize concern about avian mortality. The comparison, first collated by Erickson, et al. in 2001, seems on the surface to make a good point. But a number of major flaws exist.
Erickson, et al. report bird mortality rates at commercial wind facilities throughout the U. S., adjusted for search efficiency and carcass scavenging, ranging from 0 to 4.45 bird/ turbine/ year, and averaging 2.19. However, it is important to point out that this average is geographically biased by mortality studies in western U. S. Of the 15,000 or so turbines that were expected to be operating in North America by 2001, 3,500 were estimated to be outside of California. Approximately 510 turbines from outside of California were included in mortality studies in which Erickson, et al. derived their 2.19 figure. However, only two of these mortality studies involving 19 turbines were located east of the Mississippi River (Searsburg, VT, Somerset, PA).
Given this, it is a major oversight that Erickson, et al. failed to point out that songbird mortality at tall man-made structures is a phenomenon that is well documented in eastern North America but does not appear to be as large a phenomenon in the west (Avery et al., 1980). This has been theorized to be due to the lower density of nocturnal migration in the west and different weather patterns. Whatever the cause, figures from mortality studies in the west should not be used to project mortality rates at proposed wind sites in the east. To make this point in the extreme, one certainly would not use mortality studies from wind turbines at the North Pole to calculate the impacts of commercial wind energy on neotropical migrants in the U. S. Since the Erickson, et al. comparison was published, mortality studies at two wind plants in the east (Backbone Mountain, WV and Buffalo Mountain, TN) have shown mortality rates much higher than the Erickson, et al. derived "national average". These latter studies also indicate what had been strongly suggested by the Avery, et al. bibliography -that a higher proportion of avian mortality at commercial wind energy in the east will be from protected passerines that migrate at night.
The most egregious error in the Erickson, et al. document is that they have made their comparison too early in the build-out of commercial wind energy. To provide perspective on this, one can look at eastern North America (east of the Mississippi) where there are currently ~300 turbines (> 200-ft structures). I think everyone would agree that the avian mortality at these ~300 turbines is much less than that caused by all the autos in the east, or all the cats in the east, or all the windows in the east, or all the communications towers in the east, or all the tall buildings in the east. It is ridiculous to even make such a mortality comparison at this stage in the development of commercial wind energy.
The U. S. Department of Energy has made projections on commercial wind potential for each state. When one adds up the number of turbines to fill this projection (based on 1.5 MW/ turbine size) it is not unrealistic to see that eastern North America could support potentially half a million land based wind turbines. Erickson, et al. should have given more attention to the anticipated number of wind turbines in 2025 (rather than 2001) in comparing commercial wind power mortality to that from other sources of mortality in the east. I reiterate that future comparisons should analyze eastern U. S. separately form western U. S. Given currently documented mortality rates of about 2 to 10 birds and 2 to ~80 bats per turbine documented in the east, the projected impact of turbines in eastern U. S. could be in the range of 1-5 million birds per year and 1 - 40 million bats per year.
Of course, there are many factors that can affect these estimates. One factor is that the exact number and the dimensional characteristics of turbines in the completed build-out are unknown. Another factor is that there are so few mortality studies carried out so far, that the confidence level in the current mortality rates are very low. A third factor is that by the time the projected build-out occurs (20-30 years?), population changes for many night migrating animals are likely to decline further and thus comes the irony that less mortality would occur and therefore commercial wind turbines would be less of a problem for existing populations. The five long-term communications tower mortality studies that exist in eastern North America all showed an order of magnitude decline in annual mortality from roughly 1960 to 1980. The reason for the decline is not fully understood as this is much greater than the avian population decline during the period estimated from other monitoring methods, but population decline is suspected to be one factor contributing to this lower tower mortality.
The point here is that it appears wind turbine mortality for migrant songbirds is very likely to be at least in the range of communication tower mortality. It is certainly not low enough that we can brush it aside and careful attention needs to be applied to siting wind energy facilities because this is currently the primary means we have for mitigating avian (and bat) mortality.
Another misleading aspect of the Erickson, et al. mortality comparison is that there is no substantive consideration for the differences in species affected by the various mortality sources they consider. For example, the 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. From the evidence available, it appears large scale development of commercial wind energy in the Americas will have some, as yet unknown, impact on the population levels of sensitive songbird species such the Kirtland's Warbler, Cerulean Warbler, Golden-winged Warbler, Henslow's Sparrow, Painted Bunting and more than 100 other neotropical migrant bird species.
In summary, the proponent's ecological consultants have not provided any service by simply recanting the Erickson, et al. 2001 jargon, which is now widely seen as prematurely conceived.