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 determined 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 reproductive 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.

Why avian colonies vary in size and how food competition among nearby colonies a¡ects o¡spring quality are still not completely understood. We simultaneously examined the e¡ects of four scales of breeding density on two measures of o¡spring viability (body condition and T-cell-mediated immunity) in the colonial Magellanic penguin. Body condition of £edglings was inversely correlated with breeding density within 100 m 2 of nests, and decreased with increasing numbers of breeding pairs competing within the parental foraging ranges (100 km), probably as a result of density-dependent food depletion. The T-cell-mediated immune response was positively correlated with body condition, re£ecting, to some extent, the previous breeding-density e¡ects, and was negatively correlated with colony size, which may be related to social stress. However, given the e¡ect of protein intake on cell immunity, this result could also indicate a thus far neglected cost of coloniality, namely the consumption of low-protein food to compensate for the depletion of optimal prey. These results were not in£uenced by other traits, nor by the current exposure of birds to parasites and diseases, as measured by serological variables. Since body condition and the T-cell-mediated immune response of £edgling birds are indicators of their survival and recruitment prospects, the costs we have identi¢ed can explain variability in colony size in relation to food competition with surrounding colonies, as well as the skewed distribution toward small colonies in this species.

No abstract

No abstract

Food availability has been proposed as one of the main factors regulating population sizes in birds. Seabirds have provided evidence for the hypothesis that food depletion due to intraspecific competition explains variability in colony size. However, the predictions derived from this hypothesis have not been fully tested due mainly to the difficulties in measuring food availability in marine environments. We measured stable isotopes of nitrogen (δ15N) and carbon (δ13C) in the blood of Magellanic Penguins (Spheniscus magellanicus), which reveal information about their consumed prey and foraging habits. We tested if conspecific competition causes food depletion, affecting penguin breeding performance and, ultimately, the size of the colonies. Blood δ15N values of adults and chicks significantly decreased with increasing size of their colonies and with the number of conspecifics breeding within the parental foraging ranges. This suggests that high breeding densities provoke the depletion of high‐quality prey (mainly anchovy). We also found positive relationships between δ13C values and density of conspecifics within the parental foraging ranges, indicating that when competition for food is high, individuals tend to feed closer to the colony on prey of lower quality. Adult δ15N values were positively correlated with breeding success at the colonies, which was negatively correlated with the density of conspecifics within foraging ranges. Moreover, δ15N values of fledglings were positively correlated with their body condition but not with their T‐cell mediated immune response considered as two measures of their survival prospects. This decreased breeding output was translated to the colony‐size structure of the population, since colony size was negatively correlated with the number of conspecifics breeding within the parental foraging ranges. Therefore, we provide strong evidence suggesting that density‐dependent food depletion determines the distribution of colony sizes in birds.

No abstract

No abstract

Abstract.-Magellanic Penguins (Spheniscus magellanicus) show little sexual dimorphism, and although males are usually larger than females, sexing by direct observation may be difficult, especially in the case of chicks. In this paper we evaluate the utility of four different PCR-based sex determination techniques using genomic DNA for sexing Magellanic Penguins. We found that the primer set designed for sex determination in Collared Flycatcher (Ficedula albicollis) also provided a reliable, simple and convenient sexing procedure for Magellanic Penguins. Additionally, we obtained discriminant functions for sexing adults and chicks, sampled at six colonies differing in size and other ecological characteristics. Discriminant function for adults used two variables, bill length and bill depth that correctly classified 97% of the birds. Discriminant function for chicks included bill length and flipper length and correctly classified 78% of the individuals. Although molecular and biometric approaches could be useful for sexing adult Magellanic Penguins, only molecular procedures proved appropriate for accurately sexing chicks.