Diet and trophic position of Leach’s storm-petrel Oceanodroma leucorhoa during breeding and moult, inferred from stable isotope analysis of feathers

We combined conventional dietary sampling (2002) with stable isotope analysis of nitrogen (δ15N) and carbon (δ13C) in feathers of Leach’s storm-petrels Oceanodroma leucorhoa (2001 and 2002) to investigate temporal, sexand age-related variation in diet and trophic position, at Baccalieu Island, Newfoundland, Canada. Nestlings are fed mainly fish and crustaceans (ca. 90 and 9% by mass, respectively) toward the end of chick-rearing. The bulk of the fish were vertically migrating myctophids, associated with the offshore environment. Crustaceans were mainly parasitic hyperiid amphipods Hyperia galba, commonly found nearshore in association with scyphozoan jellyfish hosts. δ15N values in summer-grown feathers were similar between years, but differed between age-classes in 2001 when chicks occupied slightly higher trophic level (3.2 vs. 3.1). δ13C values in summer-grown feathers were lower for chicks than adults. Predictions from a 3-source dual-isotope linear mixing model indicated that adults consumed 47% glacier lanternfish Benthosema glaciale, 5% capelin Mallotus villosis and 49% H. galba in 2001, while in 2002 respective values were 87, 6 and 7%. Model output for chicks was not biologically meaningful, possibly due to the lipid-rich (and δ13C depleted) stomach oil fed to chicks by adults. For adults, no sex differences were found in values of either isotope during summer. δ15N values were significantly higher in feathers grown during summer than during mid-moult, suggesting increased consumption of crustaceans in winter. δ13C values in adult primaries were relatively enriched, consistent with movement of post-breeding birds toward tropical waters. Combining conventional and chemical analyses has provided some of the first insights into the year-round ecology of the smallest and most abundant seabird breeding in eastern North America.