Non-breeding behaviour in the Brown Skua (Stercorarius antarcticus lonnbergi): insights from modelling moulting patterns and stable isotope analyses

ABSTRACT

Long-term changes in the life history and behaviour of seabirds during the non-breeding season can reflect shifts in environmental conditions. However, long-term marine studies are scarce, particularly on southern hemisphere seabirds. Here, we used moult scores from 86 Brown Skuas (Stercorarius antarcticus lonnbergi), a large predatory seabird breeding on the Chatham Islands, Aotearoa/New Zealand to model both the timing and duration of primary feather moult. In addition, we analysed stable isotope values (δ¹³C and δ¹⁵N) from 62 modern (2014–16) and ten museum tail feathers. These data provide insights into the non-breeding behaviour of Brown Skua. Interestingly, our results show that the primary feather moult occurred prior to birds departing the colony, starting on average on 2 January ± 5 days (SE). The average start of primary feather moult occurred five days prior to the end of breeding (7 January ± 10 days (SD)) and 42 days before the birds departed the colony (13 February ± 11 days (SD)). The average duration of primary feather moult was 189 ± 14 days (SE). Importantly, low δ¹³C values in four females suggested that tail feather moult might also occur while skuas are at the colony. There was no difference in tail feather δ¹³C and δ¹⁵N values between any pairwise comparison of modern and museum years. However, values of δ¹⁵N from tail feathers sampled in 2014 were different from those sampled in 2015 and 2016. This large annual variation in δ¹⁵N values from tail feathers over such a short period makes long-term comparisons difficult to interpret, particularly between years with low sample sizes. While the stable isotope analyses of tail feathers are informative, we recommend future studies of skuas sample the primary coverts rather than tail feathers.

KEYWORDS:

• Diet
• δ¹³C and δ¹⁵N
• migration
• seabird
• Stercorariidae

Acknowledgements

We thank the Hokotehi Moriori Trust and Ngāti Mutunga o Wharekauri Iwi for their support; Department of Conservation for help with logistics, specifically Tansy Bliss, and Connie Norgate; Ian Southey for help with collecting feathers; Josette Delgado, Anna Kilimnik and Julie Brown for conducting the isotope analyses; Vivian Ward for assistance with graphics; Rob van Bemmelen and Vincent Mourik for helpful comments on skua moult and analyses; Edin Whitehead, Dave Boyle, Jemma Welch, Jamie Cooper, Leon Berard, Oscar Thomas and Dave Houston for providing images. We thank the Editor, Associate Editor and the three anonymous reviewers for their helpful comments, which greatly improved the manuscript. All fieldwork and experiments were undertaken in accordance with the New Zealand Wildlife Act authority 39406-FAU and The University of Auckland Animal Ethics permit 001382.

Disclosure statement

No potential conflict of interest was reported by the authors.

Supplementary material

Supplemental data for this article can be accessed at https://doi.org/10.1080/01584197.2022.2161914.

Additional information

Funding

This work was supported by a New Zealand International Doctoral Research Scholarship; The University of Auckland; the Birds New Zealand Research Fund and the James Sharon Watson Conservation Trust.