Inter-annual variability in diet during crèche (December to February) over 22 years (1989 to 2010) was examined for gentoo penguins Pygoscelis papua breeding at Bird Island, South Georgia (54 º0’S, 38º2’W). Overall, diets comprised 51 % crustaceans and 49 % fish by mass. Crustaceans were present in 89 % of samples and were the main prey (> 50 % by mass) in 10 years of the study. Antarctic krill Euphausia superba were present in 85 % of all diet samples. Fish were present in 79 % of samples and were the main prey in 12 years, with Champsocephalus gunnari and Lepidonotothen larseni the most frequently recorded species, in 51 % and 33 % of samples respectively. The energy or mass of krill in the diet was the most reliable predictor of breeding success (the number of chicks fledged per breeding pair); the correlation between model-predicted and observed values was 0.58. We compared annual patterns of gentoo penguin diet variability with those of macaroni penguins Eudyptes chrysolophus breeding at the same location. Our results suggest that the availability of krill is a key source of diet variability for both species, but their diets indicate that gentoo penguins are generalist predators (feeding on pelagic and bentho-pelagic prey), while macaroni penguins are krill specialists (feeding on pelagic prey). Differences in the response to variability in key prey species is an important factor separating the ecological niches of these two sympatric krill predators.