An important aspect of foraging ecology is the extent to which different individuals orgenders within a population exploit food resources in a different manner. For diving seabirds, muchof this information relates either to short-term dietary data or indirect measures such as time budgets.Moreover, dietary specialisation can be difficult to detect due to biases associated with conventionalsampling techniques. We used stable isotope ratios in blood and feathers to infer trophic and habitatspecialisations among 4 diving seabird taxa—the gentoo penguin Pygoscelis papua, the macaronipenguin Eudyptes chrysolophus, the South Georgian shag Phalacrocorax (atriceps) georgianus andthe Kerguelen shag P. (atriceps) verrucosus. This allowed us to investigate foraging specialisationand assess whether social dominance or differences in foraging preferences explained the observedpatterns. In all taxa where sexes were known we found that males foraged at a higher trophic level(δ15N values) than females, although this was not significant in macaroni penguins. We believe thatthis is linked to a dual foraging strategy among female macaroni penguins. For South Georgianshags, we found that sex-related dietary differences persisted for long periods (inferred from stableisotope analyses of feathers and blood). We suggest that the trophic differences are driven by differencesin physiological performance, with males tending to dive deeper than females because of theirlarger size, and hence able to access higher trophic level prey items. Moreover, male and femaleshags tend to forage at different times of day; therefore, social dominance by males is unlikely to bedriving the observed differences. We also recorded highly significant relationships between stableisotope signatures in blood (representing the breeding season diet) and those in feathers (mostly representingthe previous non-breeding season diet) in both the South Georgian and Kerguelen shags.This strongly suggests that these 2 taxa include individuals with distinct foraging specialisation (andmost probably foraging locations) that are maintained over long periods.