The diet of the Antarctic Prion Pachyptila desolata was examined using food samples regurgitated by adult birds during five breeding seasons at Bird Island, South Georgia. In all years the diet was mainly crustaceans, with a small proportion of myctophid fish and trace amounts of ephalopods. Antarctic krill Euphausia superba was the dominant prey item in three years and was replaced by calanoid copepods, especially Rhincalanus gigas and Calanoides acutus, in two years of low krill abundance. Differences in the prey species taken and observations of foraging behaviour suggest that in years of low krill availability Antarctic Prions forage closer inshore, taking copepods by filtering surface water through their palatal lamellae. By switching to feeding on copepods Antarctic Prions are apparently able to maintain a comparable level of reproductive success, unlike most other krill feeding species which suffer much reduced reproductive performance in years of reduced krill availability.