DOES PREY PREFERENCE AFFECT HABITAT CHOICE IN ANTARCTIC SEABIRDS

Diet composition of the members of 2 seabird species assemblages in the Scotia-Weddell Confluence region, Antarctica, was investigated during 3 seasons/years: spring 1983, autumn 1986, and winter 1988. One assemblage frequented the pack ice and the other was present in adjacent open waters; most members of the latter species assemblage vacated the Antarctic during winter. We sought answers to 2 questions: (1) Did the 2 species assemblages depend on food webs that differed substantially; and (2) Was there a trophic basis to explain why the pack-ice assemblage did not expand into the open water left vacant by the other during winter? To test an a priori hypothesis that diet was affected by habitat, diet samples were obtained from birds encountered in 3 habitats: open water, sparse concentrations of ice, and heavy ice cover. Cluster analysis showed broad overlap in seabird diet regardless of species, habitat (ice/water mass) or year. Seabirds exploited prey largely according to ranked availability, although they appeared to choose the larger fish and crustaceans over smaller crustaceans. Myctophids in particular, but also krill and squid, were the main prey groups. Diets did not separate on the basis of predator/prey size to any appreciable degree even though a 1000-fold difference in predator size existed. Feeding success, as indicated by fullness of stomach, of the members of the 2 species assemblages was highest when in their respective usual habitats. We conclude that open-water seabirds lack the specialized foraging behaviors required to exploit the pack-ice environment that open waters during the winter offer much poorer feeding conditions than those of the pack ice.