Annual variation in great skua diets: the importance of commercial fisheries and predation on seabirds revealed by combining dietary analyses

This study combines conventional dietary assessment with stable isotope techniques to describe Great Skua (Stercorarius skua) diet. Stable carbon and nitrogen isotope ratios in feathers of Great Skua chicks collected over three years were analyzed in conjunction with pellet and regurgitate collections. A significant drop in trophic status was detected in 1997, which likely resulted from an increase in herring and mackerel in the diet. These items were almost certainly obtained from a commercial trawler, as evidenced by a significant drop in territorial attendance during the ship's absence. Feathers yielded significantly different δ13C values among years, and part of this may have been related to a period of enhanced phytoplankton growth during 1996. A combined approach, such as the one described here, is likely to become increasingly useful in elucidating the diets of polyphagous birds.