Abstract
The main aim of the EOWDC Bird Collision Avoidance Study has been to improve our understanding of seabird flight behaviour inside an offshore wind farm. This should be achieved through collection of as detailed seabird flight data as possible rather than through estimation of avoidance rates for collision risk modelling per se. The focus is on seabird flight behaviour during the breeding period and post-breeding period when densities are highest in the Aberdeen area. The technical improvements of the monitoring equipment employed in the Aberdeen Offshore Wind Farm made it possible to track seabirds inside the array and measure meso-avoidance more confidently than before. It has been possible to match video camera recordings of seabird movements to a sample of their radar tracks. A total of 1,753 coupled tracks were recorded during 2020 and 1,370 tracks during 2021, which was beyond expectations and formed the basis for robust assessments of flight behaviours of target species in different parts of the wind farm array. The target sample size for species-specific meso-avoidance of 250 was reached for all key species, and the target for micro-avoidance of 100 was reached for herring gull (Larus argentatus) and black-legged kittiwake (Rissa tridactyla, hereafter referred to as kittiwake). The level of mesoavoidance recorded was 0.5 for kittiwakes, 0.7 for herring gulls and 0.5 for Northern gannets (Morus bassanus, hereafter referred to as gannet) and great black-backed gulls (Larus marinus). Together with the recorded high levels of micro-avoidance in all target species (> 0.96) it is now evident that seabirds will be exposed to very low risks of collision in offshore wind farms during daylight hours. This was also substantiated by the fact that no collisions or even narrow escapes were recorded in over 10,000 bird videos during the two years of monitoring covering the April – October period.
The results from the EOWDC study strongly indicate that the within wind farm avoidance response of the studied species of seabirds towards turbines mainly takes place within 100-120 m distance from rotors and that the response intensifies as the seabirds approach the rotor blades. In proximity to the rotors the recorded meso-avoidance response behaviour for all four species was manifested as a complex 3-dimensional pattern. Commuting gannets appeared to reduce flight altitude, whereas herring gulls and kittiwakes displayed a slight increase in mean flight height as they approached the rotor blades. When assessing the recorded flight orientation of the birds relative to the rotors commuting gannets and kittiwakes appeared to deflect around 80 m distance from the rotors and herring gulls at 50 m. The flight models revealed that turbulence and wind speed had the strongest effect on the profiles of flight behaviour of all target species. The pattern of responsive flight behaviour seemed to break down during situations with strong turbulence, while wind speed mainly affected the distance at which the increase in flight height took place. The trends resolved by the flight models were apparent irrespective of whether the birds were recorded as feeding or commuting.