title: Population variation in seabird non-breeding season ecology: implications for vulnerability assessments
Creators: name: Buckingham, Lila
Publisher: name: University of Liverpool Repository
Year: 2025

description: For animals that breed in temperate and polar regions, the highest levels of mortality typically occur during the non-breeding season, driven by harsh environmental conditions that increase energy expenditure and reduce food availability. Despite this crucial link with mortality, our understanding of non-breeding season distribution, behaviour, diet and energetics remains poor. Seabird populations have declined rapidly over the past century and are vulnerable to multiple marine threats. Offshore wind farms (OWFs) are an emerging threat that can impact seabirds via collision and displacement, with implications for survival and productivity. Despite the importance of understanding population-level vulnerability to OWFs, current non-breeding season assessments use observational data that are not linked to breeding colony. Broadening our understanding of population variation in exposure to and potential impact severity of OWFs is key to improving our assessment of seabird vulnerability at the population and meta-population levels. The aims of this thesis were to investigate the non-breeding season ecology of common guillemots Uria aalge and razorbills Alca torda and their vulnerability to displacement effects from OWFs. Using data collected from birds that bred at twelve colonies in the UK, I investigated species- and population-level differences in non-breeding season distribution, diet, behaviour and energy expenditure, and combined these data to assess the vulnerability of each species to displacement effects. Firstly, I used geolocation-immersion loggers to map distributions of guillemots and razorbills from each breeding colony during two key periods of the non-breeding season (post-breeding moult and mid-winter) and investigated the level of population aggregation in each species. Secondly, using dual-deployed geolocators and time-depth recorders on guillemots from four colonies, I investigated colony-level variation in monthly distribution, foraging behaviour, activity budgets and energy expenditure throughout the non-breeding season and used stable isotope analysis to infer relative trophic level during the post-breeding moult. Finally, I calculated spatial density and energetic extraction of all adult breeding birds within our tracked range throughout the non-breeding season. I found higher levels of population aggregation in razorbills than guillemots, particularly during mid-winter. Razorbills are therefore less likely to interact with OWFs than guillemots, but the impact of any overlap may be more severe at the meta-population level. Although guillemots displayed colony-level variation in distribution, diet and behaviour, they had remarkably similar energy expenditure across colonies. Energy expenditure peaked in late winter, indicating that displacement effects may have a more severe impact at this time. I identified the waters off the north-east coast of Scotland as an area of vulnerability to displacement effects in both species, as there was overlap between areas of high density and energy extraction with potential OWFs. The findings of this thesis have expanded our understanding of population variation in non-breeding season seabird ecology and have direct applications to marine spatial planning, particularly with regards to development of OWFs in UK waters.