The diet of a coastal omnivorous avian predator as revealed by stable isotope analysis

Context Little ravens (Corvus mellori) have emerged as a prominent predator on the clutches of burrow-nesting little penguins (Eudyptula minor) in recent decades on Phillip Island, Australia, raising concerns for conservation and eco-tourism. It has been suggested that consumption of penguin clutches may vary between individuals and years, with some individual diets likely dominated by penguin in some years. Aims We investigated raven energy assimilation in the world’s largest little penguin colony using stable isotopes. Methods δ13C and δ15N were extracted from feathers of marked individuals and potential food sources (marine and terrestrial items) to explore the energy assimilated by individual ravens. Key results Ravens had large variation in food resource use. We found dietary differences with age and year, with the proportional contribution of dietary sources varying more in juveniles and fledglings compared with adults and differing between sampling years. Penguin, mouse and other mammals contributed substantially to the energy assimilated by ravens. There was no evidence of variation in penguin consumption with age or year. This was true for both ‘culprits’ and ‘non-culprits’ (i.e. those that had not been observationally detected consuming penguin), which showed little difference in their dietary composition and little dietary specialisation between individuals. The invasive house mouse Mus musculus formed a prominent component of raven diet in some but not all years. Ravens prey on penguin eggs and chicks, and persist within penguin colonies where they consume a variety of food types. Conclusions The energy assimilation we describe infers a generalist diet, which in the years and birds we sampled included conservation-sensitive prey in the form of penguin. Implications We failed to detect a subset of individuals which specialised on consuming penguin. Selective removal of problematic individuals therefore appears unfeasible.