The expanding development of offshore wind farms brings a growing concern about the human impact on seabirds. The North Sea is an area were bird-wind farm interactions are assumed to increase in the coming years, but the effect on seabirds breeding on the nearby shores is not fully understood. In this thesis, we study bird flight at sea in relation to the environment to investigate the possible effects on offshore wind farm interactions. We use automated bird radar to observe birds flying inside and outside two offshore wind farms near the Dutch coast. We explore general patterns of flight and bird abundance as well as thermal soaring as this behaviour can increase collision risk in birds. We found offshore bird abundance in the breeding season is highest during the day and peaks after sunrise, and additionally increases throughout the breeding season. We also observed thermal soaring both inside and outside the wind farm on radar, and bio-logging of lesser black-backed gulls showed an increase in overlap with rotor altitude, which increases collision risk. The behaviour occurred mostly with larger temperature differences between the sea surface and the air above. Additionally, although we assumed bird flight would be affected within the wind farm, we measured no difference in average ground speed, tortuosity, or flight direction relative to the wind for birds flying inside the wind farm compared to outside. Lastly, we developed a post-processing framework for automated bird radar data to facilitate accessibility of bird radar data and comparative analyses of bird movement in space and time.

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