Ecological drivers of population size and marine distribution of storm-petrels in the northeast Atlantic

The magnitude and causes of population change and marine distributions of very small seabirds have been difficult to study, but some populations appear to be in decline. It is important to understand the range of threats that these species face, to be able to identify potential conservation actions. In this thesis I use new developments in tracking technology and statistical analysis methods to address these issues in the two smallest North Atlantic seabirds: the European Storm-petrel Hydrobates pelagicus and the Leach’s Storm-petrel Hydrobates leucorhous. A census of two of the largest Leach’s Storm-petrel colonies in the northeast Atlantic, analysed using both traditional and novel statistical methods, confirmed a continuing decline (68% across 19 years) on St Kilda, Western Isles, Scotland, and identified a substantial decline on Elliðaey, Vestmannaeyjar archipelago, Iceland (40–49% across 27 years). Dissection of Great Skua Stercorarius skua pellets from St Kilda and Mousa, Shetland, quantified the number of pellets produced per individual storm-petrel consumed. This ratio varies between storm-petrel species and between sites. These data indicate that the number of Leach’s Storm-petrels consumed by Great Skuas on St Kilda is higher than previously estimated using bioenergetics models, and represents a substantial proportion of the Leach’s Storm-petrels present on St Kilda. GPS tracking of European Storm-petrels breeding on Mousa indicates that their movements are driven primarily by avoidance of predation and intraspecific competition. Foraging areas differed between years, but in all years foraging trips were focused on the relatively shallow shelf waters to the southeast of Shetland. These findings provide novel insights into the marine distributions, at-sea behaviours, foraging ecology, and population change among storm-petrels in the northeast Atlantic, and provide an important basis for identifying marine protected areas and conservation interventions on land