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INTRODUCTION
This report, submitted to the Scientific Review Committee (SRC) by the Monitoring Team (MT), describes the results of avian fatality monitoring of approximately 2500 wind turbines conducted at Altamont Pass Wind Resource Area (APWRA) between October 2005 and October 2007. We analyzed data from the first two years of this study to calculate average annual mortality rates and an APWRA-wide fatality estimate for all species as well as for four raptor species (golden eagle, red-tailed hawk, American kestrel, and burrowing owl) separately and combined. We then analyzed the results for inter-annual and seasonal variations. We derived these estimates using methods and criteria that make them comparable to baseline mortality estimates, provided by the SRC, that were derived from earlier studies (Smallwood and Thelander 2004).
The APWRA supports a broad diversity of bird species, both resident and migratory, that regularly moves through wind turbine area (Orloff and Flannery 1996). Diurnal raptors (eagles and hawks), in particular, use the prevailing winds and updrafts for soaring and gliding during daily movement, foraging and migration. Birds passing through the rotor plane of operating wind turbines are often killed (Howell and DiDonato 1991, Orloff and Flannery 1996, Howell 1997, Smallwood and Thelander 2004). Multiple studies of the bird fatality rates at APWRA show golden eagles, red-tailed hawks, American kestrels, burrowing owls, barn owls and a diverse mix of other non-raptor species are killed each year (Howell and DiDonato 1991, Orloff and Flannery 1996, Howell 1997, Smallwood and Thelander 2004) in turbine-related incidents. Most of these species are protected by both federal and state wildlife legislation.
The current management goal for the APWRA is to significantly and substantially reduce the fatalities of birds resulting from collisions with the wind turbines and other turbine-related incidents. The principal short-term management objective is to reduce the fatalities of four high-impacted raptor species (golden eagle, red-tailed hawk, American kestrel, and burrowing owl) by 50% (of the baseline) by November 2009 through management actions including, but not limited to, a two month shut-down of turbines during the low wind season, and shutdown, removal and/or relocation of turbines previously characterized as high-risk turbines (Smallwood and Thelander 2004, Smallwood and Spiegel 2005a, 2005b, 2005c). The 50% reduction criteria is based on the 2007 Settlement Agreement between the Wind Power Companies, Alameda County, Californians for Renewable Energy, and Golden Gate Audubon Society. The management actions are outlined in Exhibit G-1 of the Settlement Agreement.
DISCUSSION
The results of this study show an apparent continued trend of high bird fatalities, both raptors and non-raptors at APWRA. The number of annual fatalities does not appear to be decreasing despite implementation of specific conservations measures including the cross-over winter shutdown program, high risk turbine removal and blade-painting. Indeed, the number of fatalities across most species increased in 2006-2007 over the previous year. The reasons for this recent trend are not yet known, but could potentially be due to the effects of stochastic ecological events on local bird abundance and movement patterns. Only continued long-term monitoring at APWRA will provide the necessary data on bird population changes required for fully understanding the causation of bird fatalities at the site.
Within the scope of the present study, as noted in the results section, there are also a variety of problems associated with this study that may affect the precision and quality of the mortality rate and fatality estimates presented in this report, some of which we will try to deal with during the report revision.
First, there is a lack of an accurate metric by which to reliably measure mortality. Estimating fatalities per turbine or megawatt (MW) for the entire AWPRA is confounded by the extreme variability in the number of functional turbines and, consequently the rated capacity, year to year and season to season (or even daily) throughout the study. Some sites in the AWPRA have vastly different operation times when compared to other sites. As a case in point, the entire Santa Clara site was shutdown for a whole year between January 2006 and February 2007. Similarly, Enertech turbines are shutdown for the rainy season independent of mitigation winter shutdowns.
Without an accurate accounting of the number of functional turbines or capacity per month or quarter, we have had to employ a static number of turbines or MWs as a metric that does not truly represent variability or dynamics of power generation as it relates to avian mortality over time. A better and more precise estimate could be gained by employing power output (kilowatt hours) as the standard metric. The seasonal or annual variability in power output may be a major factor in the observed seasonal and annual variation in avian mortality. Calculating the number fatalities or mortality rate per KW hour will give a clearer and more precise picture of the true avian cost of power production as well as enable a more accurate cost/benefit analysis of the mitigation measures employed in the field. This revised analysis would provide the SRC with a more powerful and precise tool to maximize the effects of turbine removal or shutdown mitigation while minimizing the reduction in power production across the AWPRA.
Second, the implementation of the monitoring program during the early part of the study resulted in some gaps in fatality and bird use data collection and longer fatality search intervals than in the latter portion of the study. These anomalies may confound the analysis of the inter-annual and seasonal variability in avian mortality. Additionally, minor changes were made to the data collection sheets and a cause of death 2008 APWRA Bird Fatality Study (DRAFT) 17 January 2008 protocol was instituted during the course of the study that may require conversion of the earlier data to maintain consistency.
Third, at the beginning of the study wind smiths removed all of the carcasses they found. They were documented and reported to the WRRS independent of the monitoring survey. This protocol was changed in 2006 after which all birds/bats found by wind technicians in monitored plots, with the exception of Golden Eagles, were marked and left in the field for the monitoring crews to find.
Fourth, there was a lack of randomization associated with the study design. For example all Enertech turbines are in monitored plots, as well as all of Enxco Patterson Pass site. We suggest boot strap analysis to mitigate this issue. Some sites (North Wind and Buena Vista) that were sampled in the Baseline study were not available (restricted) for selection in our plot randomization process. Because no plots were searched in these areas during this study fatality records from those sites will have to be excised from the Baseline mortality estimate to make it comparable to our estimates.
Lastly, we propose running a sensitivity analysis on the scavenger rate and other scaling factors that appear to have a large influence on our estimates for the revised report.
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