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 euphausiids other than E. superba (Thysanoessa spp. and E. frigida) in 2000. There was no clearly dominant prey group in 1999. The five-year average proportion of E. superba has remained relatively constant at around 68 % since the mid 1990s. Prey diversity and the frequency occurrence of T. gaudichaudii both increased with a decreasing proportion of E. superba in the diet. 59 % of all diets were dominated (> 90 % by mass) by euphausiids, 15 % were dominated by fish or amphipods and 27 % were mixed, suggesting a sigmoidal functional response. 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.