Changes in the distribution and abundance of common scoter and diver species in the Horns Rev I, II and III offshore windfarm areas, Denmark, 2025:Bird distribution responses to wind farms, Horns Rev

Between February 2000 and April 2025, 62 observer-based aerial surveys of birds were conducted at Horns Rev, an area of the North Sea off central Jutland, using a Distance Sampling line transect survey design. The survey area covered the offshore wind farm (OWF) areas of Horns Rev I (HR I), Horns Rev II (HR II) and Horns Rev III (HR III). The surveys were classified into four main phases according to the construction and operation of the three wind farms developed within the area. Phase 0 included 15 surveys prior to any wind farm construction, Phase 1 included 25 surveys post-construction HR I and pre-construction HR II and HR III, Phase 2 included 10 surveys post-construction HR I and HR II, but pre-construction HR III, while Phase 3 included 12 surveys post-construction of all three OWF’s. Owing to changes in survey coverage and expansion in geographic ranges during Phase 1, which could bias the results, Phase 1* was created to represent the latter surveys from November 2005 to late spring 2007. The combined data set collected between 2000 and 2025 offers a unique opportunity to address the potential change in the displacement of birds over time, based on empirical data.<br/><br/>This report describes the changes in abundance and distribution of common scoter Melanitta nigra and divers (predominantly red-throated diver Gavia stellata but potentially including some black-throated divers Gavia arctica) over the period, based on the statistical analysis of visual aerial survey data in the Danish North Sea Horns Rev area gathered during the surveys described above. Species-specific Distance Sampling analyses were undertaken to correct for various aspects of avian detection probabilities, pooled across surveys. This was since followed by survey-specific spatial analyses with covariates including water depth (bathymetry), distance from the coast and/or a geographical covariate to model the distributions of common scoters and divers at a fine geographical scale for each survey.