Abstract
From 2021 to 2023, a comprehensive birds survey were conducted for the North Sea Energy Island site in the northern central part of the Danish North Sea. In combination with the somewhat limited existing data on birds in the area, this survey aimed to gather thorough background data for future environmental impact assessments of upcoming wind farm projects. The investigation area in the North Sea consists of tidal, exposed, saline waters 22 – 60 m deep and is among the most oceanic of the Danish waters. It covered a total area of 4,814 km2. The investigation area is referred to as ‘the extended bird survey area’ and included the phase 1 area with a buffer of 20 m around it. The originally designated pre-investigation area is contained within the extended bird survey area, see Figure 1.1.
Twelve aerial bird surveys were conducted from March 2022 to November 2023 to quantify the abundance, distribution and trends of relevant birds at sea. Each survey was conducted by a single aircraft over a single day. The surveys employed the distance sampling survey method. This approach allowed for modelling selected bird species' total abundances and distributions. Based on these modelled estimates, persistency maps for the surveys area were created, highlighting areas of high or low importance for specific species or species groups across all surveys. Data from each of the twelve surveys were used to derive information for the following species/species groups: northern fulmar, northern gannet, black-legged kittiwake, and razorbill/common guillemot. The maximum estimated numbers per survey were 2,364 northern fulmars, 3,797 northern gannets, 4,967 black-legged kittiwakes, and 27,245 razorbills/common guillemots. The aerial survey revealed that the avifauna within the extended bird survey area was dominated by offshore bird species, with northern fulmar, northern gannet, black-legged kittiwake, razorbill and common guillemot being the most numerous. Other gull species than black-legged kittiwake, such as European herring gull, lesser black-backed gull and great black-backed gull were also frequently recorded in the area. Terns were recorded in low numbers. Arctic tern was most abundant, while common tern occurred in lower numbers. Terns are migratory species and not present in Danish waters over the winter.
Eleven ship-based observations of birds were conducted from November 2021 to November 2023 with special focus on species flight altitudes and flight patterns.
Nearly all individuals of some species groups, such as alcids, terns and skuas, were recorded flying very low over the sea surface (0-25 m). In contrast, waders and gulls tended to fly at higher altitudes. For example, European herring gull and great black-backed gull were recorded flying up to 214 m and 184 m above the sea surface, respectively. Observations from these surveys also provided valuable insights into species composition for birds that are challenging to identify from aerial surveys. For example, divers were exclusively red-throated divers, while alcids comprised 16% razorbills and 84% common guillemots, with notable seasonal variations in their composition.
There was from the onset of this project scheduled for recording of bird flight information from a radar system on a platform to the west of survey area with the purpose with the aim of gathering data on a) altitude distribution of birds in the area day and night and b) relative volume/movement intensity of flying birds in the vertical plane. This proved to be impractical, and the ship-based platform using rangefinder was chosen for the purpose. The flight altitudes of birds investigated from ship-based surveys represent daylight observations only and might differ from nocturnal flight patterns, which could not be measured.
An overview of existing bird data from the Danish North Sea is presented, including data from a three day aerial survey in April/May 2019, covering the entire Danish North Sea. These data were used to estimate abundances and distributions for selected species/species groups. For that data set, the following species could be used: red-throated/black-throated diver, northern fulmar, northern gannet, black-legged kittiwake and razorbill/common guillemot. These species' estimated abundances for the entire Danish North Sea in April/May 2019 were 22,648 divers, 46,437 northern fulmars, 31,723 northern gannets, 4,472 black-legged kittiwakes and 89,681 razorbills/common guillemots.