Search

We investigated how foraging habits vary among three ecologically distinct wide-ranging seabirds. Using amino acid δ15N proxies for nutrient regime (δ15NPhe) and trophic position (Δδ15NGlu-Phe), we compared Newell’s shearwater (Puffinus newelli) and Laysan albatross (Phoebastria immutabilis) foraging habits over the past 50–100 years, respectively, to published records for the Hawaiian petrel (Pterodroma sandwichensis). Standard ellipses constructed from the isotope proxies show that inter-population and interspecific foraging segregation have persisted for several decades. We found no evidence of a shift in nutrient regime at the base of the food web for the three species. However, our data identify a trophic decline during the past century for Newell’s shearwater and Laysan albatross (probability ≥ 0.97), echoing a similar decline observed in the Hawaiian petrel. During this time, Newell’s shearwaters and Hawaiian petrels have experienced population declines and Laysan albatross has experienced range extension and apparent population stability. Counting other recent studies, a pattern of trophic decline over the past century has now been identified in eight species of pelagic seabirds that breed in the Hawaiian Islands. Because our study species forage broadly across the North Pacific Ocean and differ in morphological and behavioral traits and feeding methods, the identified trophic declines suggest a pervasive shift in food web architecture within the past century.

Isotope data from Hawaiian Petrel (Pterodroma sandwichensis) primaries P1 and P6 were compared to determine whether foraging habits change between the beginning and middle of the nonbreeding season. P6 data did not differ between samples derived from a longitudinal and a minimally invasive protocol and point samples taken from the feather bases. While P6 δ13C increased longitudinally, no δ15N longitudinal trends emerged, yet inter-individual δ15N variability was high. P6 δ13C data suggest that Hawaiian Petrels molt at low latitudes. Among colonies, all of which are located in the Hawaiian Islands, USA, low P6 δ15N values for Maui birds relative to Hawaii and Lanai birds reflect foraging segregation and differential utilization of 15N-enriched oceanic regions. For the Hawaiian Petrel, the isotopic similarity between P1 and P6 indicates that analogous ecological interpretations can be drawn from these feathers, and similar foraging habits persist from the beginning to middle of the nonbreeding season. Prolonged inter-colony foraging segregation may facilitate coexistence of colonies and, together with high intra-colony foraging diversity, may reduce extinction risk for the endangered Hawaiian Petrel.