Feeding ecology of two congeneric seabirds at the South Shetland Islands, Antarctica

Abstract

Seabirds play a key role in maintaining the balance and functioning of marine ecosystems. Changing energy demands during reproduction, coupled with prey availability and competition, shape seabird foraging behaviors and diets. Brown Skuas Stercorarius antarcticus lonnbergi and South Polar Skuas S. maccormicki are opportunistic feeders and scavengers that breed sympatrically on the Antarctic Peninsula and nearby islands. In shared nesting areas, Brown Skuas often monopolize resources on land, compelling South Polar Skuas to exploit alternative trophic pathways at sea. This study investigates the diets of Brown and South Polar Skuas by analyzing regurgitated pellets (n=443) collected during three breeding seasons (2021–2023) and blood stable isotopes of carbon (δ13C) and nitrogen (δ15N) sampled in 2021/22 at the Potter Peninsula, 25 de Mayo/King George Island, South Shetland Islands, Antarctica. Pellet analysis showed that Brown Skuas consumed penguins more frequently and had a broader dietary niche compared to South Polar Skuas, which consumed more large, mesopelagic fish (predominantly Antarctic Lanternfish Electrona antarctica) and Storm Petrels. As the breeding season progressed, both species increasingly relied on marine resources from incubation to the chick-rearing, indicating dietary plasticity. Blood δ13C values suggested greater coastal/inshore foraging for Brown Skuas and offshore foraging for South Polar Skuas. Elevated δ15N values in Brown Skuas indicated a higher trophic level, and their isotopic niche breadth was wider than South Polar Skuas. No sex-related variations were detected in blood isotopic signatures. The dietary diversity, adaptability and the presence of distinct ecological niches likely facilitate skua coexistence during sympatric breeding.