Diet of adult and immature imperial cormorants, Leucocarbo atriceps, from southern Patagonia. A combined dietary approach and an exploratory analysis of stable isotopes of pellet membrane

Abstract

The use of combined conventional and stable isotope analyses (SIA) to study the diet of seabirds has become very frequent. Unfortunately, information on the trophic ecology of immature seabirds remains scarce because the sampling of tissues to perform SIA is often very limited due to the difficulty to capture these birds. The koilin membrane, which covers the regurgitated pellet casts of some seabirds, could offer an interesting non-intrusive alternative tissue to perform SIA. In this two-year study of the diet of imperial cormorants in southern Patagonia, we (1) compare the diets of breeding and non-breeding birds through conventional pellet analysis; (2) describe the diet of breeding adults and chicks through a combined method of pellet analysis and whole blood SIA; and finally, (3) compare SIA values of breeding adults obtained from blood with those obtained from koilin membranes. We found that immature individuals incorporated abundant invertebrate taxons, compared with breeding adults which relied mainly on fish. Younger and inexperienced individuals, which are not under the constraint to feed chicks, are feeding on more predictable, but lower trophic and less energetic prey. By comparing the stable isotope values of koilin and blood in breeding adults, a correlated interannual difference between the two seasons was found. Under the light of our results, the koilin offers an encouraging alternative to blood in the study of trophic ecology, particularly for ages or stages in which capture is not possible.