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We analysed summer diet and fledging mass of macaroni penguins Eudyptes chrysolophus breeding at Bird Island, South Georgia, during the crèche period (January and February) between 1989 and 2010. Crustaceans were the main prey accounting, for over 90% of the diet by mass. Antarctic krill Euphausia superba was the main prey, in 17 out of 22 years. Amphipods Themisto gaudichaudii were the main prey in 1994 and 2009, fish in 2004, and the euphausiids Thysanoessa spp. and Euphausia frigida in 2000. There was no clearly dominant prey group in 1999. Prey diversity and the frequency occurrence of T. gaudichaudii both increased with a decreasing proportion of E. superba in the diet. There was strong evidence that macaroni penguins have a sigmoidal functional response, indicating that this kind of response should be accounted for when devising ecosystem-based management reference points for seabirds. The energy and mass of all euphausiids combined (rather than E. superba in particular) in the diet were the most reliable predictors of chick fledging mass; the correlation between model-predicted and observed values was 0.84. The gross energy content of individual meals was often above average in years when the diets contained fewer euphausiids, but fledging mass was always below average in these years. Although macaroni penguins are able to feed on a variety of prey types, chick growth was always severely impacted by a shortage of euphausiids due to higher energy or time costs associated with feeding on alternative prey types. Given their reliance on euphausiids, macaroni penguins would be particularly vulnerable to potential climate-driven declines in krill stocks.\u

Diet variability and reproductive performance of macaroni penguins Eudyptes chrysolophus at Bird Island, South Georgia