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 grea t skuas Catharacta skua from 2 northeast Atlantic colonies as sampling unit s. Different feather types and blood were taken to represent intake over di fferent temporal scales. Mercury concentrations and stable isotope signatur es of these tissues were determined. General linear models demonstrated tha t trophic status, as indicated by delta(15)N, had an influence on tissue me rcury concentrations. However this effect was relatively minor compared to that of foraging area. Samples of the same feather types from the same indi viduals in consecutive years suggest that a factor other than dietary speci alisation and foraging area is of major importance in determining intra-spe cific variability in mercury levels. It was concluded that there are a numb er of interacting factors contributing to intra-specific variability in mer cury levels and the relative importance of these factors varies both spatia lly and temporally.