Nicholas School of the Environment and Earth Sciences
Bird Site Assessments
Assessments of Existing Wind Sites
The first step in site assessment is to determine which turbine areas will be searched at regular intervals. This is usually a large circular area around the base of a turbine (i.e. 50 m at Tehachape). The frequency can vary dramatically based on the purpose and duration of the study. The Tehachapi study not only included a random sampling of turbines, but also those on the ends of strings to determine any separate effects of that placement. Second, searcher effectiveness experiments are conducted to determine the success rate of each searcher at carcass detection. These are controlled experiments where someone other than the searcher places carcasses in a random fashion around the search area and the success rate is then used to extrapolate how many carcasses are not found during the study searches. Lastly a carcass removal rate should be done at each site. To do this researchers place bird carcasses in the search area and visit each day to see how many have been deteriorated or removed by other animals. These three calculations are used in conjunction with the actual carcass searches to determine the average number of bird kills for the entire wind farm (Anderson et al, 2004).
Site Assessment: Nocturnal Bird Migration Over an Appalachian Ridge at a Proposed Wind Power Project
As a specific example of one such avian study, a nocturnal investigation was done in the area of the proposed Mt. Storm wind power project in West Virginia. This project will consist of approximately 150-200 turbines to total 300 MW of capacity. The turbines will be located along the Allegheny Front ridgeline and requires avian studies to determine if birds use this ridgeline to navigate and the height at which most birds fly through this area.
Many avian fatalities at modern wind power developments disproportionably affect nocturnal migrants. For this reason the study chooses to see how this population is affected at this site. The mobile X-band radar technology is well suited for studying low-altitude migration (1.5 km above ground level) at wind power development sites and is uniquely able to provide local information about bird flight altitudes, flight direction, behavior, and passages rates for an approximate 1.5 km radius.
It is important that the results of avian studies not be extrapolated to different times of the year, areas or geographies with different species makeup. The results of this particular study are specific to the autumn of 2003 when the data was collected.
At this site passage rates increased approximately 1–2 hours after sunset, leveled off, and then decreased slightly later in the evening. These observations seem to be consistent with other studies in which the intensity of nocturnal migration begins to increase approximately 30–60 minutes after sunset, peaks around midnight, and declines steadily thereafter until dawn. A high daily variation in migration passage rates also makes it important to study when peaks may correspond with events such as the passage of weather fronts. The study also used two near by sample sites due to the sensitive nature of passage rates (they can vary significantly just 10km away). The study area at Allegheny Ridge appeared to have higher rates of migration compared to other locations where the researchers have conducted studies using similar equipment and methods. As the height of the birds’ flight is considered, only 13-16% of flights were recorded at elevations 150 m and below, where they would be in the same territory as the wind turbines. There was large among-night variation in flight altitudes though m ean flight altitudes were always above the maximal proposed turbine heights. Interestingly however, there were 5 nights when mean flight altitudes fell between 200 and 300 m above the ground. Daily variation in flight altitudes probably reflected changes in both species composition and vertical structure of the atmosphere and weather.
The conclusion for this study, as for most, does not necessarily make a recommendation about whether or not the wind farm should be placed in this area. It does not appear that a set of rules or standards exists to either promote or reject the placement of a wind farm due to an avian study. The main take away from this study is that most nocturnal migrants did not follow along the Allegheny Front ridgeline during autumn migration. This study also promoted the use of portable marine radars as tools for collecting important baseline avian information.