Abstract
In 2024 and 2025, the second year (Y2) of ornithological studies were conducted to provide baseline data for the Environmental Impact Assessment (EIA) of future offshore wind farms in the NSI area, located in the eastern part of the Danish North Sea. These surveys are continuations of the first survey year (Y1) conducted from April 2023 to March 2024 and aimed to gather background data for future environmental impact assessments related to upcoming offshore wind farm projects.
The ornithological studies consisted of two main components. The first component aimed to provide data on bird species abundances and distributions across the pre-investigation area and a 20 km buffer zone around it, referred to as the survey area, over the course of a year. To achieve this, eight aerial surveys were conducted during Y2. During these surveys, a total of eight species groups and 27 species of birds were observed in the North Sea I area. Each survey was performed using two aircraft and employed the distance sampling survey method. This approach allowed for the modelling of total abundances and distributions of selected bird species. Based on these modelled estimates, a persistency map for the survey area was created, highlighting areas of high or low importance for specific species or species groups across all surveys. These persistence models were performed for combined Y1 and Y2 data sets in this report. Data from each of the eight surveys from Y2 were used to derive information about the abundances and distributions of the following species/species groups: red-throated/black-throated diver, northern gannet, little gull, black-legged kittiwake, and razorbill/common guillemot. The maximum bird numbers per survey were 4,401 divers, 4,145 northern gannets, 4,869 little gulls, 11,659 black-legged kittiwakes, and 26,953 razorbills/common guillemots. When comparing Y1 and Y2 aerial surveys, overall patterns of species distribution were broadly consistent, with core areas of use for key species persisting across years. However, Y2 benefited from more favourable weather conditions, improved visibility, and greater spatial coverage, resulting in higher data quality and fewer survey gaps. These factors contributed to increased detection rates for several species and provided a more complete seasonal representation of bird presence and distribution. While some differences in abundance between years may reflect genuine ecological changes, such as shifts in migration patterns or local environmental conditions, others are likely influenced by the enhanced survey effort and improved observation conditions in Y2.
The other part aimed to describe the movements characterise the movements of flying birds within the NSI pre-investigation area. Observations from Y1 and Y2 provided data on species composition, migration patterns, and flight altitude distributions for selected species and species groups. Bird migration through the area was seasonally structured, with considerable variability between years and observation positions. Migration intensity peaked during autumn (September to October), particularly in the second survey year, indicating that inter-annual differences may be influenced by large-scale environmental factors or shifts in population dynamics and species composition. A consistent diurnal pattern was evident, with peak migration activity occurring shortly after sunrise. Directional movements generally followed expected sea-sonal trends, with northward migration in spring and southward in autumn. However, local variations were observed, likely driven by weather conditions and coastal topography, with some birds adopting east southeast or southwest trajectories, especially during autumn. Furthermore, most diurnal flights occurred at low altitudes, with over 75% of individuals flying below 100 meters, particularly among auks, divers, and sea ducks. In contrast, gulls and terns displayed a broader vertical range and were the only groups frequently recorded flying above 100 meters. Species composition varied by season, year, and observation position. Spring was dominated by lesser black-backed gulls, red-throated divers, and common terns, while autumn and winter saw greater auk diversity and increased numbers of common guillemots and great black-backed gulls. Notable inter-annual changes, such as a decline in razorbills and an increase in little gulls, further highlight the dynamic and variable nature of seabird migration through the area.
Data from a vertical radar provided information on flight magnitude and altitude both day and night during Y2. The radar records objects that can be a single bird or a group of birds. Neither species nor group size can be inferred from data from the radar. There was both annual and diurnal variation in the bird flight activity. The movement of flying birds showed much higher flight activity in September and October 2024 than in any other month, relating to bird autumn migration.