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We used stable-isotope analysis (SIA) to evaluate sources of variation in the diet of and prey selection by Magellanic penguins Spheniscus magellanicus breeding on the Argentinean Patagonia coast. Our aim was to determine potential sources of variation in diet, focusing mainly on sex and age, although geographic and temporal effects were also taken into account. In addition, we evaluated how prey selection affects offspring quality. We measured stable nitrogen (δ 15 N) and carbon (δ 13 C) isotope values in whole blood of chicks (n = 98), yearlings (n = 15) and adults (n = 143) in 9 different breeding colonies during 2 consecutive breeding seasons (1999 and 2000). We also measured stable isotope val- ues in representative prey consumed by this species. Stable nitrogen and carbon isotope values in blood for penguins varied from 17.8 to 20.0‰ and from -18.1 to -14.8‰, respectively. Both colony and season had a significant effect on the 2 isotope signatures. Adult males had higher blood δ 15 N and δ 13 C values than females. Age significantly affected both δ 15 N and δ 13 C values, with chicks showing the highest and yearlings the lowest δ 15 N values. Chicks showed lower δ 13 C values than yearlings and adults. Mean prey δ 15 N values ranged from 13.6 ‰ in squid to 18.0 ‰ in octopus. As anchovy, the main prey consumed by the species in the study area, did not differ from other fish species (hake) in its δ 15 N value, we used it to represent a fish dietary alternative. Using a 2-source (anchovy and squid) isotopic mixing model, we de- termined that the mean proportion of anchovy in the diet was 49% for yearlings, 76% for chicks, and 69% and 67% for adult males and females, respectively. Sex and age differences in diet, as revealed by stable isotopes, may be the consequence of individual morphology (sexual size dimorphism) and re- productive constraints imposed by chick development since growing young require more nutritive prey than adults and yearlings. This reasoning would also explain the significant and positive correlation found between proportion of anchovy in the diet and body condition of chicks.

marine ecology. progress series

Food resource utilisation by the Magellanic penguin evaluated through stable-isotope analysis: segregation by sex and age and influence on offspring quality