Variability in mercury concentrations of great skuas Catharacta skua:: the influence of colony, diet and trophic status inferred from stable isotope signatures

A range of parameters thought to contribute to intra-specific variation in mercury levels were investigated using the feathers and blood of adult great skuas Catharacta skua from 2 northeast Atlantic colonies as sampling units. Different feather types and blood were taken to represent intake over different temporal scales. Mercury concentrations and stable isotope signatures of these tissues were determined. General linear models demonstrated that trophic status, as indicated by delta(15)N, had an influence on tissue mercury concentrations. However this effect was relatively minor compared to that of foraging area. Samples of the same feather types from the same individuals in consecutive years suggest that a factor other than dietary specialisation and foraging area is of major importance in determining intra-specific variability in mercury levels. It was concluded that there are a number of interacting factors contributing to intra-specific variability in mercury levels and the relative importance of these factors varies both spatially and temporally.