A survey of the bird populations in Oldfjällen in Jämtland county, Sweden, was initiated in 2003 prior to the construction of a wind-farm on two adjacent hills, Storrun and Frösörun by the lake Övre Oldsjön. After a gap in the surveys in 2004, comprehensive studies were continued during 2005–2008, prior to the construction. The construction-work started during autumn 2008, and the turbines were erected during 2009. Power production was started during the autumn of 2009. The post-construction bird studies were carried out in 2010 and 2011.
This report summarizes all the studies that were performed before and after the construction of the wind-farm. Many of the separate studies were made using a BACI method (Before-After-Control-Impact) enabling comparisons of the situation before and after development, using a control area. In addition, a study of collision rates and scavenger removal rates is reported.
As expected, the movement and migration of birds over Storrun was of rather limited magnitude. Although the majority of observations during the investigated periods spring and autumn are assumed to be migrating birds, larger flocks of birds are rare in the area. Ca. 20 % of all the birds that passed through or near the wind-farm area flew in heights comparable to the rotor-swept zone. However, more than 40 % of the raptors and waders passed through in that zone, which implies a collision risk. These two groups of birds have shown to be particularly prone to collisions with rotor blades in other studies.
The studies using territory mapping, showed that small passerines were dominating in the study plots. The number and densities of territories declined post-construction, but as this was even true in a nearby reference area, we were not able to relate this decline to the wind-farm as such. The largest decline was however noted in the study plot most affected by constructions, and it had a significant decline in number of territories per species.
A pair of red-throated divers that bred in the study area before the construction disappeared completely after the construction-phase. Observations of ducks, not breeding in the study area, also ceased in the territory mapping after construction. The number of territorial willow grouse also declined considerably post-construction, but as no pairs bred in the reference area, we have no comparable control. The displacement effect of roads, towers and installations could only be tested for two passerine species; the willow warbler and the tree pipit. We were not able to prove a significant displacement on either of these species.
The line-transects of birds showed a downward trend both in number of species and of individuals during the course of the study. This is in line with what was shown in the study plots using territory mapping.
The studies of waterbirds on two large blanket bogs three and ten kilometers to the east and northeast of the wind-farm showed a marked decline in numbers after the construction phase. This is in line with what is shown other places in Sweden during the same period. Because of the large distance between the areas, we do not believe that the decline has anything to do with the wind-farm.
The pre- and post-construction surveys of grouse species using trained bird-dogs showed a variation in densities of willow ptarmigan over years that were parallel to the variation in other areas in Jämtland. However, the density of capercaillie decreased considerably after the building. These birds are forest-dwelling, and it is difficult to envisage a direct effect of the wind-farm, but an indirect disturbance effect cannot be excluded.
No great snipes were found lekking in the wind-farm area, nor was this species seen during any of the other studies in the area. The effect on great snipe by wind farms has thus not been possible to investigate in this study. Young gyrfalcons that were fitted with satellite transmitters kept primarily within an area of five km radius of the nest-site after fledging and before dispersal. 50 % of the positions were within 2 km of the nest. No positions were from within Storrun wind-farm, which was 18 km from the nearest gyrfalcon territory. The most important preventive measure relative to wind-farm development and gyrfalcons is judged to be the distance between their breeding sites and turbines. No adult birds were tagged and we have thus not been able to determine the area used by them.
The initial movement pattern of young golden eagles tagged with satellite transmitters was quite similar to that of the young gyrfalcons. Before leaving their natal areas in October-November, 90 % of their positions were within a radius of 5 km from their nests, and 50 % were within 1.2 km. As for gyrfalcons, the collision risk factor will largely be determined by the distance between nest sites and turbines. The tracking showed that young birds return to their natal sites the following summer, and also the year thereafter, thus being exposed to additional collision risks. In addition, the birds use hills and outcrops in the terrain during migration to save energy using thermal updrafts. Such sites are prime targets for wind-farm developments in north and central Sweden. The area use of adult golden eagles was not studied.
The surveys of collision victims using a specially trained dog were conducted between one and three times per month during more than one year. During this time, nine verified collision victims were found, whereof four willow grouse. An additional willow grouse was found by wind-farm personnel shortly after the study was terminated. The dog also found 14 remains of willow grouse that could not be verified as collision victims. No birds of prey were found, but three red crossbills, one fieldfare, and one redwing were found. A collision between a willow grouse and a turbine tower was eye-witnessed by us, strengthening the assumption that this species collides with the tower structures, not the rotor blades.
To provide a measure of the rate of disappearance of collision victims caused by scavengers, dead birds of different grouse species were placed in the terrain around the turbines during five bouts. The results from this experiment enabled us to build a model to adjust for the removal rate, thereby providing a better estimate for the true number of collisions. For willow grouse, this was calculated to be 0.5 victims per turbine and year. If the additional unverified remains are included, the collision rate of willow grouse rises to ca. 1.25 bird per turbine and year.
Fågelundersökningar vid Storruns vindkraftanläggning, Jämtland (Bird surveys at Storrun's wind farm, Jämtland) is only available in Swedish.