Foods and Aspects of Growth in the Antarctic Petrel and Southern Fulmar Breeding at Hop Island, Rauer Group, East Antarctica

Abstract

Foods of adult Antarctic Petrels Thalassoica antarctica and Southern Fulmars Fulmarus glacialoides breeding on Hop Island, Rauer Group, East Antarctica, were obtained using water-offloading techniques, from late incubation to late post-guard periods. Both species took mainly krill, particularly Euphausia superba, and fish, almost entirely Pleuragramma antarcticum; other crustaceans and cephalopods were infrequent. The increase of E. crystallorophias, particularly in Southern Fulmars, was associated with increased P. antarcticum in the diet of both species. No significant differences in prey sizes were detected but the smaller Antarctic Petrel tended to contain more fish and less krill. Results are compared with previous studies; all show a restricted number of prey species, and a wide range of prey sizes. Variations in proportions of specific groups may reflec sampling methods, but more likely reflect prey abundance or availability. The importance of ice cover is discussed. Inshore feeding by Southern Fulmars, and differing feeding methods, may allow segregation of foraging: assuming similar rates of digestion, the more digested food in Antarctic Petrels suggests a greater foraging range. This is supported by differences in attendance shifts at nests. Growth rates of chicks deprived a meal were compared with those of chicks given supplementary meals, and controls. Deprivation apparently caused no immediate effects; additional meals tended to increase growth of Antarctic Petrel chicks. Small sample sizes confounded interpretation but suggested that Antarctic Petrels, which typically breed further south than Southern Fulmars, are better adapted to local conditions where sympatric with Southern Fulmars. The slightly larger volumes of oil retrieved with Antarctic Petrel food supports this since oil offers a more efficient method of transporting food to growing chicks. For these two fulmarine petrels, segregation of food requirements may be by foraging differences; where sympatric, segregation of peak demands may also result from separation of laying periods.