Abstract:
Seabirds feed at high trophic levels and therefore are important indicators of marine ecosystem health. In this thesis, the year-round distribution, activity patterns and diet of Chatham Island brown skuas (Catharacta antarctica lonnbergi) were investigated - a population that is thought to be in decline. Firstly, a combination of light-based geolocation sensing and stable isotope analysis of feathers was used to investigate the non-breeding behaviour of brown skuas. By analysing data from 27 skuas that were tracked in two seasons (2015-16), it was shown that all tracked individuals dispersed throughout mixed subtropical-subantarctic and subtropical waters of the Chatham Rise between February and July. Interestingly, the duration of migration was reduced compared to brown skuas breeding at higher latitudes. Secondly, fine-scale movements and activity patterns of skuas were studied while at their breeding grounds, during incubation and early chick-rearing periods. By analysing 102 GPS tracks collected from 64 individuals in three seasons (2014-16), it was found that female and male skuas differed in their spatial distribution and diel activity levels. During the night, females mostly remained at the nest site, whereas males predominantly foraged on burrow-nesting petrels. Although both sexes spent equal times at the nest during the day, only females foraged over nearby farmland. Furthermore, the contribution of major prey species to the breeding season diet of brown skuas was estimated using a Bayesian stable isotope mixing model. Consistent with differences in movement, female and male brown skuas showed significant differences in their diet, a pattern that could be driven by differences in their parental roles. Finally, to test whether the skuas' non-breeding distribution and diet during breeding have changed over time, stable isotope data of modern and museum feathers (1871-2016) and modern and historical blood samples (1987-2016) were compared. The findings indicate no major shifts in the non-breeding distribution of brown skuas over a 145 year period, but suggest that females shifted towards a more terrestrial breeding season diet during the past 30 years, possibly as a response to changes in local farming practices and/or the marine ecosystem.