Subspecies-Selective Predation of Snow Petrels by Skuas

Abstract

Snow petrel (Pagodroma nivea) populations in Antarctica are highly variable in body size (coefficient of variation of 5%, 7%, 8%, 9% for wing, tarsus, and bill lengths, and bill depth, respectively). Two subspecies have been recognised, a large one P. n. major and a small one P. n. minor. At few locations both subspecies breed sympatrically in varying proportions. Snow petrels' main predator on breeding grounds is the south polar skua (Catharacta maccormicki). To understand the evolutionary processes shaping body size in the snow petrel, I evaluated selective predation of snow petrels by south polar skuas on an island near Antarctica. Food remains (n = 43) collected in 16 skua territories indicated no significant subspecies-selective predation on adult snow petrel. In both subspecies nest characteristics (open, intermediate and concealed; n = 110) and predation on eggs and chicks were studied. Egg predation did neither differ significantly between the larger (15.8%) and smaller subspecies (12.5%) of snow petrel, nor within each subspecies relative to body size of the pairs. More chicks issued from pairs of the larger subspecies (37.5%) than chicks issued from pairs of the smaller one (0%) disappeared during the breeding season because of predation by skuas. No size-selective predation on chicks was found within the large subspecies, Birds of the large subspecies occupied more open nest sites than birds of the small subspecies (P < 0.001), and open nests were preyed upon more often than concealed nests (P = 0.041). As body size seems to be heritable in the snow petrel according to subspecies (h 2 = 0.61 ± 0.14) and within subspecies (smaller: h 2 = 0.92 ± 0.31: larger: h 2 = 0.87 ± 0.37). predation by south polar skuas may pose a selection against the larger subspecies.