Flexible foraging strategies in a highly pelagic seabird revealed by seasonal isotopic niche variation

Flexibility in foraging strategy is an important mechanism by which seabirds cope with spatiotemporal heterogeneity in food availability and the variable energetic constraints of their annual life cycle. Foraging strategy flexibility was investigated in the grey-faced petrel Pterodroma gouldi breeding on Ihumoana Island (36 degrees 53 ' S, 174 degrees 26 ' E) using stable isotope analyses. Intra- and inter-annual variations in stable isotope values, isotopic niches and diet inferred from isotope mixing models were studied by analysing delta N-15 and delta C-13 in adult wing feathers and blood, chick down and body feathers, and muscle from spontaneously regurgitated prey, collected during 2013 and 2014 breeding seasons. Grey-faced petrels exhibited variations in stable isotopes, isotopic niches and diet more markedly throughout their annual life cycle than between years. A trophic segregation occurred between adults and chicks presumably from adults feeding inshore and chicks being fed more oceanic prey of higher trophic level. Stable-isotope mixing models revealed that adult diet during the breeding season could consist mainly of ram's horn squids Spirula spirula and chick diet of crustaceans, fish and other cephalopods being secondary prey throughout the breeding season. Adult male and female isotopic niches slightly differed. Finally, isotopic niche in adults during non-breeding was similar to that during breeding, suggesting non-breeding foraging areas located off the eastern Australian coast, around the limit between the Tasman and Coral seas. Our results demonstrated plasticity in the foraging strategy of grey-faced petrels in response to the changing nutritional demands of their annual cycle and to changes in oceanographic conditions likely driven by El Nino Southern Oscillation.

Automated summary

Flexibility in foraging strategy is an important mechanism for grey-faced petrels to adapt to spatiotemporal heterogeneity in food availability and energetic constraints of their annual life cycle. Stable isotope analyses revealed variations in stable isotopes, isotopic niches, and diet throughout the annual life cycle. Trophic segregation occurred between adults and chicks, with adults feeding inshore and chicks consuming more oceanic prey. The results demonstrated the plasticity of foraging strategy in response to changing nutritional demands and oceanographic conditions.