Use of stable isotopes to quantify seasonal changes of trophic niche and levels of population and individual specialisation in seabirds

Niche variation is a widespread phenomenon that has important implications for ecological interactions and conservation biology, but few studies have quantified the trophic niche width (TNW) and its within- and between-individual components. We used stable isotopes of body feathers to investigate (1) seasonal isotopic niche changes of 4 southern procellariiforms and, (2) the level to which individuals are specialised relative to their population within each season. (1) delta C-13 and delta N-15 values of chicks and adults indicated a well-defined trophic segregation within the seabird assemblage during both the breeding and poorly known moulting (inter-nesting) periods, and they underlined marked species-specific seasonal changes. One species (light-mantled sooty albatross) remained within the Southern Ocean both during breeding and body feather moult, while the 3 others migrated either to oceanic subtropical waters (wandering and sooty albatrosses) or high isotopic marine areas (e.g. productive neritic waters; white-chinned petrel) during the moulting period. (2) Isotopic variances and TNW were generally low, indicating that birds belong to isotopic specialist populations within a given period. Variances and TNW were larger for white-chinned petrel chicks and light-mantled sooty albatross adults, respectively, indicating isotopic generalist populations and revealing 2 new foraging behaviours. Individual white-chinned petrel chicks segregated by their feather isotopic signatures, thus indicating individual/pair specialisation of parent birds during the chick-rearing period. The light-mantled sooty albatross population included mostly isotopic generalist individuals during the moulting period, with individuals using different habitats and diets. The study highlights the utility of feather isotopic signature for determining TNW and points out the necessity to develop such new approaches to better depict niche variations at both population and individual levels.