In 2007 the report “Changes in bird habitat utilisation around the Horns Rev 1 offshore wind farm, with particular emphasis on Common Scoter” was prepared by NERI as commissioned by Vattenfall A/S. This report analysed recent (post-construction) changes in the distribution of Common Scoter in the area within and around the Horns Rev 1 wind farm. This report compared the survey data from the original studies, with those collected during four surveys carried out in early 2007, but no attempt was made to estimate the total numbers present (using distance sampling techniques) nor to generate density estimates at a high spatial resolution using spatial modelling techniques.
The present report utilised the survey data from early 2007 to derive modelled abundance and distribution estimates for Common Scoter. This was performed across the entire study area for each of the four surveys performed in early 2007. The estimated total numbers of Common Scoters in January to April 2007 were 223,122, 269,341, 189,282 and 67,058 birds.
Within the area of the Horns Rev 2 wind farm site the estimated number of Common Scoter for each of the above surveys were 16,131, 25,041, 16,958 and 9,099 birds on 25 January, 15 February, 3 March and 1 April respectively.
It was difficult to assess the magnitude of reduction in Common Scoter density in and near the Horns Rev 2 wind farm site based on experiences from Horn Rev 1. Despite appearances of initial displacement at Horns Rev 1, within 4 years of construction, densities within the wind farm did not differ significantly from those outside, which may suggest some temporal adjustment on behalf of the birds. On the other hand, we cannot exclude the possibility that these patterns observed at Horns Rev 1 were the results of responses to food supply, and had feeding been suitable, the birds would have foraged between the turbines from immediately after construction. Given the uncertainty surrounding the observed responses of the birds, it was decided to offer a series of differing scenarios for displacement to assess the relative effect on the total numbers of individuals present and in relation to established levels for international importance for the flyway population of this species.
The potential number of displaced Common Scoters from the Horns Rev 2 wind farm site was calculated on the basis of a series of four differing scenarios, assuming displacement of 5, 10, 25 and 50% of the birds present within the wind farm site, and with a linear decreasing impact away from the wind farm periphery out to a distance of 500 m. This was performed for each of the four survey data sets. Under these assumptions a calculated total number of 1,528 (5% reduction), 3,055 (10% reduction), 7,638 (25% reduction) and 15,277 (50% reduction) Common Scoters would be displaced with a distribution pattern as found on 15 February 2007 when most birds were estimated present in the wind farm area. In the ornithological impact assessment for the Horns Rev 2 a 100% displacement scenario was assumed, based on the present knowledge from the Horns Rev 1 wind farm. Results from early 2007 showed a temporal adjustment to the Horns Rev 1 wind farm site. Thus the estimated numbers of displaced Common Scoters are considerably lower than presented in the ornithological impact assessment for Horns Rev 2 in 2006.
It is considered unlikely that substantial numbers of Common Scoters will be displaced from the Horns Rev 2 site post construction, assuming that they show no new response to the construction of the turbines and that the food supply there remains intact. Nevertheless, it is important to stress that whilst the numbers estimated within the Horns Rev 2 impact area constitute between 7 and 14% of the numbers of Common Scoter in the entire study area during early 2007, their absolute numbers exceeded the criteria for international importance (16,000 individuals) in all surveys except that of April 2007.