At-sea foraging behaviour in Hutton’s shearwater (Puffinus huttoni) as revealed by stable isotope analysis

Stable isotope analysis of feathers can provide an indirect method to investigate the diet and foraging locations of birds during the time the feathers were growing. We used the isotopic composition of experimentally-induced feathers to investigate the foraging locations of the Hutton’s shearwater (Puffinus huttoni), an endangered seabird that is a breeding endemic to the Kaikōura region of New Zealand. The isotopic composition of feathers was first compared with potential prey items collected from the near-shore marine environment near the breeding colony. By applying trophic fractionation factors (2–4 ‰ increase in δ15N for every 1 ‰ increase in δ13C) and comparing the isotopic composition of the induced tail feathers and sampled prey items, we found that feather isotopic compositions were not consistent with a diet based on feeding locally. Both the δ13C and δ15N from zooplankton and fish collected within 8 km of Kaikōura were significantly different than the isotopic composition of induced feathers and were outside of the range expected for consumed local prey items. Instead, we found the isotopic composition of Hutton’s shearwater feathers was more consistent with feeding on potential prey items in the seas north-east and around Banks Peninsula, an area c. 100 km south of the breeding colony and where they had been tracked previously. Stable isotope analysis can provide insight into the foraging behaviour of birds at sea and demonstrates the importance of isotopic research in pinpointing foraging locations in seabirds with large geographic ranges.