A 6 megawatt, 11 turbine wind power development was constructed by Green Mountain Power Corporation in Searsburg, southern Vermont, in 1996. The turbines are Zond Z-40 turbines that stand 197 feet (about 60 m) above the ground (to the rotor tip) on tubular towers. To determine whether birds were impacted, a series of modified BA (Before, After) studies was conducted before construction (1993-1996), during (1996), and after (1997) construction on the project site. The studies were designed to monitor changes in breeding bird community (species composition and abundance) on the site, examine the behavior and numbers of songbirds migrating at night over the site and hawks migrating over the site in daylight, and search for carcasses of birds that might have collided with the turbines. Findings of the study are as follows.
- A literature search was conducted to determine the extent and diversity of bird fatalities associated with tall structures (wind turbines, towers, stacks, and buildings) in the eastern United States and Canada. In addition, the literature search examined the impact of ceilometers and other types of lights on avian behavior and fatalities at tall structures. The survey revealed an abundance of tower kill studies with few being conducted recently. There were only two studies that reported carcass searches at wind power facilities in the eastern United States and Canada.
- Breeding bird surveys (point counts taken at 21 points) were conducted in 1994 before construction of the turbines, 1996 during construction, and 1997 after construction. Although no major changes in species composition were found, the numbers of several interior forest breeding birds were lower after construction than before construction and several edge species were more numerous after construction. It is possible that the songs of some of these species could not be heard because of turbine noise during some surveys. Such effects may be the result of forest fragmentation. Further study could determine if interior forest species recover as roadsides and areas around turbines are reforested and to determine if at the same time edge species decline.
- Searches for nesting diurnal raptors, particularly Northern Goshawk, conducted in spring of 1994 revealed no raptors nesting on or adjacent to the turbine site. No evidence of raptors nesting on the site was found during breeding bird surveys in 1996 and 1997, although two sightings of Sharp-shinned Hawks within 4 km of the site suggest that this species nests nearby.
- Nocturnal migration of songbirds through the wind power facility during spring 1994 and 1997 and autumn 1996 and 1997 suggested that the site is not a predominant migratory pathway. The numbers of birds flying over the site were the same as, or less than, the numbers reported from other inland locations in New England and many fewer than reported from studies done farther south. Fewer migrants were counted after construction of the turbines, perhaps indicating avoidance of the immediate turbine area by migrants.
- Hawk migration counts taken in 1993, 1994, 1996, and 1997 revealed small numbers of these migrants. Numbers of hawks counted were lower or the same as most sites in New England and two orders of magnitude lower than the counts taken at such concentration locations as Cape May, New Jersey, Lighthouse Point, Connecticut, and Hawk Mountain, Pennsylvania. A small proportion of the hawks observed prior to construction flew near enough to the turbine area to be at risk. Fewer hawks were counted in the year after the turbines were constructed than in the years prior to construction, perhaps indicating avoidance behavior.
- Searches for dead birds were conducted adjacent to turbines during the period June through October. No carcasses were recovered. Scavenging was rare with some songbird carcasses (from road and window kills) remaining on the ground for two or more months. Two tests of observer efficiency revealed that the two observers found about 50% of songbird carcasses placed out at random.
Overall, results of the studies reported herein suggest that the Searsburg, Vermont wind power facility does not pose a major threat to avian populations that breed on the site or migrate through the site. However, fewer interior forest breeding songbirds were heard singing in the area immediately surrounding the turbines. This effect may be transitory in that these birds may habituate and recolonize as the sites are partially reforested. However, until this is demonstrated, this disturbance should be recognized as a potential impact of this type of development, especially in northeastern forests.