title: The year-round importance of breeding sites in common guillemots Uria aalge
Creators: name: Bennett, Sophie
Publisher: name: University of Liverpool Repository
Year: 2024

description: As many natural populations are declining and facing increasing threats, understanding the mechanisms governing key demographic rates such as breeding success is increasingly critical. Across heterogeneous environments, animal breeding sites may differ in their quality such that some sites offer an intrinsically higher likelihood of breeding success than others. In declining populations, average quality of breeding sites used by the population is predicted to increase as individuals preferentially occupy the best locations (the ‘buffer effect’). However, we currently have limited understanding of whether this regulatory process operates in populations, or how its effects may vary in populations showing highly dynamic changes in density over time. Furthermore, the competition for higher quality breeding sites may often be intense such that individuals invest considerable time in site occupancy to protect higher quality breeding sites, even outside the breeding season. The site defence hypothesis predicts that occupancy should be positively correlated with breeding timing and success such that more successful sites are occupied earlier and/or more frequently, and result in higher breeding success. However, few studies test these predictions in populations where non-breeding season site occupancy dominates the annual cycle, limiting our understanding of potential fitness consequences. Even fewer studies investigate drivers and consequences of individual variation in site occupancy in the non-breeding season, hindering our understanding of how this behaviour may affect individual and population level fitness. In this thesis, I use a combination of long-term population demography data, time-lapse photography and novel inferences from biologging data to investigate how the quality of breeding sites may provide resilience to populations undergoing environmental change and drive the year-round behaviour of common guillemots, Uria aalge, a widespread colonial seabird species. The data in this thesis originate from a single colony, the Isle of May, East Scotland. Throughout, I consider higher quality sites to be those with a higher average historic breeding success across the long-term data time series. Firstly, using a 38-year dataset of site occupancy, I demonstrate that, in the breeding season, higher quality breeding sites are more likely to be occupied across changes in population size and trend, including when the population was in periods of decline and subsequent recovery. I then show that a disproportionate use of higher-quality breeding sites drives population average site quality and breeding success. Hence, I find evidence for site-dependent regulation of breeding success operating through site quality. Next, using time-lapse photography to record occupancy, I test whether the quality of breeding sites influences occupancy in the non-breeding season (October- March). I find support for the site-defence hypothesis; higher quality sites are occupied earlier, more often and for longer daily durations. Further, those sites that are occupied earlier are more likely to have a successful breeding attempt in the following breeding season. Importantly, I demonstrate that the first date a site was occupied in the non-breeding season is a stronger predictor of breeding success than lay date (a well-established and widespread predictor of breeding success) despite occurring up to seven months earlier. Lastly, I investigate why, given these benefits, individuals vary in their frequency and pattern of occupancy in the non-breeding season using biologging data. I demonstrate considerable inter-individual variation in occupancy and find that occupancy return and duration of absences from the colony had a negative relationship with distance travelled. Finally, I demonstrate that those individuals occupying sites for longer were more efficient foragers, indicating that they may be of higher quality than other individuals. Overall, I demonstrate that the quality of breeding sites can regulate population level fitness, and drive the behaviour of guillemots throughout the year. Crucially, I show how population and site level breeding success may be influenced by both extrinsic factors (the quality of a breeding site), and intrinsic factors (the ability of individuals to obtain and occupy a higher quality breeding site) that ultimately act to increase population resilience to changes in population size and trend.