The Hawaiian petrel (Pterodroma sandwichensis) is an endangered seabird nesting solely on the We are using a stable isotope approach to gain insight into Hawaiian petrel feeding ecology. Specifically, carbon and nitrogen isotope values (δ13C and δ15N) are being used to study variation in trophic level and foraging location among populations and throughout the annual cycle. Because isotope values in different tissues reflect prey items assimilated over unique periods of time, our multi-tissue approach should reveal any differences in feeding ecology during the breeding versus non-breeding season. Results are also being compared with a companion genetic study utilizing the same individuals. Results/Conclusions Preliminary data from feathers collected during our first field season demonstrate significant variation in stable isotope values between Hawaiian petrel colonies. For example, average δ15N values for colonies on the islands of Kauai and Thus, our preliminary data suggest that populations of the Hawaiian petrel are ecologically, as well as genetically differentiated. Isotopic analyses of muscle and claw from salvaged birds (reflective of diet during the breeding season) may provide further insight, including variation between populations during the summer breeding season and differences between adult and juvenile diet. Given the existence of only one Hawaiian petrel diet study, we believe our analyses will provide critical information for the successful conservation of this endangered species.