Stable isotopes reveal variable foraging behaviour in a colony of the Imperial Shag Phalacrocorax atriceps: differences between ages, sexes and years

Abstract

Stable isotope analysis (SIA) is used in the study of trophic relationships in food webs, being also a powerful tool for the study of intraspecific diet segregation. Unlike short-term data from most conventional diet studies, SIA can also provide information about times when sea-birds stay out at sea and are thus not easily accessible. Imperial Shags Phalacrocorax atriceps are resident sea-birds showing sexual dimorphism in body size. Previous studies showed strong intra-species dietary segregation between male and female Imperial Shags during the breeding season. Between 2006 and 2009 at New Island in the Falkland Islands/Islas Malvinas (southwest Atlantic), we investigated if intraspecific differences were also present between age-groups, namely adults and immature males. We further tested if differences existed over the year, particularly during the non-breeding season. Finally, we tested if differences were consistent among studied years. We found no differences in delta C-13 values between immature and adult males. However, lower delta N-15 values indicated feeding at lower trophic levels for immatures. This might be explained by poorer hunting abilities of young, inexperienced birds, compared to adults. In both the breeding and non-breeding seasons, differences among years in both delta C-13 and delta N-15 suggest that the foraging behaviour of the shags differed among years in terms of spatial distribution and trophic level. Males consistently foraged on a higher trophic level than females and had lower delta C-13 values, which is in line with the use of foraging areas further offshore.