The Horns Rev 2 offshore wind farm comprises 91 turbines covering a total area of 34 km2 ca. 14 km northwest of the Horns Rev 1 offshore wind farm and started its operational phase in September 2009. DONG Energy contracted Aarhus University, DCE (Danish Centre for Environment and Energy) to undertake a total of 10 aerial surveys of birds throughout the Horns Rev study area during the winters and springs of 2011 and 2012. These surveys were conducted using the same survey methods as used during the Horns Rev 2 pre-construction surveys to offer a before/after comparison of bird distribution within and around the impacted area. The single most abundant bird species in the study area was Common Scoter, which utilized the shallower waters of the surveyed area. Up to 187,000 individuals were observed during a single survey in March 2011 making the area internationally importance for this species. Divers were also present in the study area in significant numbers, of which Red-throated Diver comprised more than 90% of all diver observations. The most abundant gull species was Herring Gull, but Kittiwake and Little Gull were also frequently recorded within the area. Razorbills and Guillemots were mostly recorded in the western parts of the study area. In this report, particular emphasis has been placed upon analyses of potential changes in the abundance and distribution of Common Scoters and diver species (i.e. Red-throated Diver/Black-throated Diver observations combined) before and after construction of turbines. In collaboration with DMP Statistical Solutions UK Ltd. in St. Andrews we used distance sampling tools and spatial adaptive modelling to estimate densities and predict distribution surfaces for the two species/species groups. For the purpose of the pre- and post-construction comparisons, 10 surveys conducted in the survey area between November 2005 and April 2007 were selected. Abundance and distribution data from these 10 surveys were compared to data from the 10 post-construction surveys presented in this report. Aerial surveys conducted in relation to Horns Rev 1 between 2000 and 2004 covered an area that only included parts of the Horns Rev 2 wind farm site, and were thus not included. A series of six surveys performed during the winter of 2007/2008 were delivered for this analysis with a data structure that was not compatible with the selected surveys, and were thus omitted. Although the overall abundance of divers was similar during the pre- and post-construction periods, marked distributional changes within the survey area were found. While densities significantly increased in the westernmost parts of the study area, significant decreases were found in and around the Horns Rev 2 offshore wind farm. While the cause for decreased densities at distances of 10 km from the Horns Rev wind farm are unclear, there are clear indications of a wind farm related displacement in areas closer to the wind farm. The distance from the wind farm towards the post-construction high density area for divers west of the wind farm was the best indication of a wind farm related disturbance effect. This distance was 5-6 km. The mean reduction in diver numbers within the area of the turbines of the Horns Rev 2 wind farm was estimated to be 17 individuals, and 48 birds when calculat6 ing reductions in the wind farm plus a buffer zone of 2 km around the outermost turbines. Taking the total area around the wind farm in which post-construction abundance was significantly lower than the preconstruction abundance, there was a mean of 173 fewer individuals. Significant reductions in density in the north eastern parts of the study area are unlikely to be related to the presence of the wind farm. Overall Common Scoter abundance was also similar comparing the pre- and post-construction periods, but again marked distributional changes were found. Most notably Common Scoter abundances decreased in the area of approximately 100 km2 around the Horns Rev 2 offshore wind farm and in the coastal area west of Skallingen post-construction. Densities increased in areas south of the Horns Rev 1 offshore wind farm, east of the Horns Rev 2 wind farm and in the western and northwestern parts of the survey area post-construction. Although there is no obvious explanation for reductions in scoter density in the eastern parts of the study area, decreases in density that correspond to the area in which the Horns Rev 2 offshore wind farm was constructed are likely to be associated to the presence of the wind farm. The abundance of Common Scoter decreased by a mean of 5,901 birds within the Horns Rev 2 wind farm, equivalent to 1.1% of the flyway population. Mean abundance post-construction was 13,193 birds less than preconstruction within the wind farms and a buffer zone of 2 km around the outer turbines. The mean difference in abundance pre- and post-construction in the ca. 50 km2 area around the wind farm in which significant reductions were found amounted to 10,996 birds fewer, equivalent to 2.0% of the flyway population. There were no significant changes in bird densities within the Horns Rev 1 offshore wind farm area across the 20 surveys. Horns Rev 1 was already operational during both the pre- and post-construction surveys conducted in relation to this work, and thus no change in utilization of the Horns Rev 1 wind farm site was expected or found over this period.