Regional variations in the diet of the South Polar Skua (Stercorarius maccormicki) in the Ross Sea region, Antarctica

The South Polar Skua (Stercorarius maccormicki) is an opportunistic feeder, predator, and scavenger widely distributed in the coastal ecosystems of Antarctica. However, although some studies have explored its foraging behavior, many aspects, including regional variations, remain insufficiently understood. Thus, this study aimed to characterize regional variations in the dietary composition of South Polar Skuas breeding at sites along the Northern Victoria Land coast in the Ross Sea, where environmental conditions vary markedly among colonies. The dietary composition and foraging characteristics of the South Polar Skua populations in the Ross Sea region were investigated by analyzing stable carbon (δ¹³C) and nitrogen (δ¹⁵N) isotopes in blood samples. The values were then used to quantify isotopic niches and estimate dietary contributions. Results revealed significant regional variations in the dietary composition of skuas. Compared with skuas at other sites, skuas at Cape Möbius showed higher δ¹³C values, suggesting a greater reliance on fish and the placenta or carcasses of Weddell seals for food. While δ¹⁵N values did not significantly differ among skuas at various sites, skuas at Cape Möbius and Cape Washington had higher values than those at other sites, suggesting access to higher trophic-level prey. Site-specific dietary contributions were revealed, with Adélie Penguin eggs and fish dominating the diet at Cape Hallett and Inexpressible Island, whereas Emperor Penguin eggs and fish were more prominent at Cape Washington. Our findings demonstrate that the species exhibits site-specific foraging patterns shaped by local prey availability. These results provide novel insights into the trophic ecology of this top predator and contribute to a broader understanding of the spatial dietary variation in Antarctic seabirds.