Seabird year-round and historical feeding ecology: blood and feather δ13C and δ15N values document foraging plasticity of small sympatric petrels

The foraging ecology of small seabirds remains poorly understood because of the difficulty of studying them at sea. Here, the extent to which 3 sympatric seabirds (blue petrel, thin-billed prion and common diving petrel) alter their foraging ecology across the annual cycle was investigated using stable isotopes. delta C-13 and delta N-15 values were used as proxies of the birds' foraging habitat and diet, respectively, and were measured in 3 tissues (plasma, blood cells and feathers) that record trophic information at different time scales. Long-term temporal changes were investigated by measuring feather isotopic values from museum specimens. The study was conducted at the subantarctic Kerguelen Islands and emphasizes 4 main features. (1) The 3 species highlight a strong connection between subantarctic and Antarctic pelagic ecosystems, because they all foraged in Antarctic waters at some stages of the annual cycle. (2) Foraging niches are stage-dependent, with petrels shifting their feeding grounds during reproduction either from oceanic to productive coastal waters (common diving petrel) or from subantarctic to high-Antarctic waters where they fed primarily on crustaceans (blue petrel and thin-billed prion). (3) The common diving petrel segregated from the surface-feeders blue petrel and thin-billed prion by a coastal habitat and lower trophic level prey, while the blue petrel segregated from the thin-billed prion by foraging further south and including more fish in its diet. (4) Feather delta C-13 and delta N-15 values from historical and recent specimens of thin-billed prion depicted a pronounced temporal shift to higher latitudes in its main moulting ground, where it feeds on higher trophic level prey. The study contributes to growing evidence that seabirds exhibit considerable foraging plasticity and sheds new light on their flexibility at different time scales (from intra-seasonal to decadal).