Feeding ecology of the endangered Hawaiian petrel: Insights from stable isotope analysis

Background/Question/Methods

The Hawaiian petrel (Pterodroma sandwichensis) is an endangered seabird nesting solely on the Hawaiian Islands and foraging throughout the eastern North Pacific (from 0-50° N). Like many seabirds, knowledge of the Hawaiian petrel’s feeding ecology is limited by logistical difficulties associated with studying rare, highly-mobile marine organisms. Knowledge of feeding ecology is critical for Hawaiian petrel conservation because reproductive success may depend upon the availability of high trophic level prey (suitable in energy content and nutritional value) and energy expenditure associated with foraging. Additionally, there is a need to understand inter-population ecological variability as conservation managers consider allocation of their time and resources among Hawaiian petrel colonies.

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 Hawaii differ by 3.4 ‰ (one-way ANOVA, p=0.01) with Kauai and Hawaii averaging 12 ‰ and 15.5 ‰, respectively. Both δ¹⁵N and δ¹³C values are consistent with Hawaii birds wintering near the cold tongue of the North Pacific (where denitrification leads to elevated δ¹⁵N values throughout the food web), and Kauai birds wintering in locations near the Hawaiian Islands. Differences in stable isotope values are mirrored by the genetic data, which show significant differentiation between Kauai and Hawaii (pairwise F_st=0.41).

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.