| Summary: | Michalik A, McGill RAR, van Noordwijk HJ, et al. Stable isotopes reveal variable foraging behaviour in a colony of the Imperial Shag Phalacrocorax atriceps: differences between ages, sexes and years. Journal of Ornithology . 2013;154(1):239-249. 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 seabirds stay out at sea and are thus not easily accessible. Imperial Shags Phalacrocorax atriceps are resident seabirds 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 δ13C values between immature and adult males. However, lower δ15N 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 δ13C and δ15N 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 δ13C values, which is in line with the use of foraging areas further offshore.
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