Techniques to link individual migration patterns of seabirds with diet specialization, condition and breeding performance

Relationships between seabird breeding and food supply have been subject to much study, but we know little about the influence of food supply in winter on survival and subsequent breeding performance of seabirds, or about the extent to which migrations and winter home ranges of seabirds are influenced by food abundance. The Great Skua Stercorarius skua is a colonially breeding seabird that displays a wide range of feeding techniques during and outside the breeding season. We deployed satellite PTTs and data loggers to determine migration routes and winter quarters of breeding adult Great Skuas. We also studied dietary preferences by direct observations and by use of fatty acid and stable isotope signatures, and measured body condition and breeding performance of individually colour marked birds. Our objective was to develop methods to investigate whether winter foraging conditions may influence subsequent breeding performance, and to collect data to assess whether migrations and winter ranges differed between individuals in ways that may relate to fitness. We found large differences in the winter home ranges of individual Great Skuas, with some wintering off Iberia and others off west Africa. The latter behaviour was unexpected and may represent a new habit to exploit waste from fisheries that have recently developed on the west African continental shelf. Fatty acid signatures provide a clear signal of the presence of demersal fish from fishery discards in skua diet and can be used to assess the feeding preference of individuals. Stable isotopes of C and N in primary feathers show changes in diet of individuals through the period of moult, which is thought to be from September to December. These approaches show that combining the use of data loggers and sampling of forensic markers of colour ringed adults permits the winter diet of individuals to be related to their geographic location in winter and to their subsequent breeding performance.