This report presents data from four aerial surveys of birds in the Horns Rev wind farm area in 2004. Three surveys from the winter and spring of 2004 are thoroughly reported here. The fourth survey of 9 September 2004 is reported in general terms, but not included in presentations of distribution and effect analyses of the wind farm. Data from this survey will be thoroughly dealt with in a future report. Including the four surveys of 2004, a total of 29 surveys have been performed in that area since August 1999.
The four surveys in 2004 were performed on 29 February, 26 March, 10 May and 9 September.
The operational phase of the wind farm commenced in 2002. Hence nine surveys from 2003 and 2004 have been conducted during the postconstruction period. In order to achieve maximal compatibility between pre- and post-construction data sets, most analyses were comparing surveys done in February through May of 2000 and 2001 (seven surveys) and 2003 and 2004 (six surveys).
Common Scoter was by far the most numerous bird species in the study area in 2004 as well as during the previous years of investigations. More than 95,000 individuals were observed in 2004. Herring Gull was the second most numerous species in 2004, with more than 4,400 birds observed. Divers, Gannet, Eiders, Little Gull, Arctic/Common Tern and Guillemot/Razorbill were relatively regular in the study area, and are treated in detail in this report.
Divers, Common Scoter and Guillemots/Razorbills showed an increased avoidance of the wind farm area (and zones within 2 and 4 km of it) after the erection of the wind turbines. In contrast, Herring Gulls, Little Gulls and Arctic/Common Terns showed a decreased avoidance of the wind farm area.
Common Scoter showed a difference in the spatial distribution within the study area in 2004 compared to previous years. Shallow offshore areas west of the wind farm became important to the species, with occurrence of birds in the westernmost parts of the survey area. Furthermore, the seasonal shift in offshore appearance of Common Scoter described in previous reports seems to have changed into a more permanent presence during winter. This leads to the hypothesis that a food resource has formed in these areas, which was not present during the pre-construction period.
Given the apparent general changes in Common Scoter distribution in the study area comparison of pre- and post-construction distribution analyses for this species must be interpreted with caution. Despite this the data strongly indicate that common scoters respond to the presence of the wind turbines by avoidance of that area. The area southeast of the wind farm, previously used by Common Scoter and particularly in February through April, became less attractive to the species. Simultaneously areas west and north of the wind farm, with previously very few Common Scoters, supported greater numbers of this species, while only very few birds were recorded within the wind farm.
The reason for the change in avoidance of the wind farm area for divers, Common Scoter and Guillemot/Razorbill is unknown. Disturbance effects from the wind turbines is one possible reason. Disturbance from increased human activity associated with maintenance of the wind turbines could be another. But changes in the distribution of food resources in the study area could potentially play a role too.
The change in gull and tern preference for the wind farm area is likely to have been caused by the presence of the wind turbines and the associated boat activity in the area.