The energetic costs of animal movement change with body condition, although the consequences of this for foraging efficiency are rarely considered. We deployed externally attached devices to Magellanic penguins (Spheniscus magellanicus), known to increase the costs of swimming via increased drag in a consistent manner, and noted, however, that foraging behaviour and efficiency varied dramatically between years. We used our results to construct an energetics-based model and found that small increases in drag due to the attachment of externally attached tags predicted accelerating harm as prey availability decreased, which accorded with our observations. This explains earlier observations of differential breeding success in tagged versus untagged penguins in particular years, highlights the importance of understanding how animal body condition may affect population processes in general and advocates caution in interpretations of tag-derived data over variable environmental conditions.

The stomach contents of juvenile Magellanic penguins, Spheniscus magellanicus, stranded along the Atlantic coast (21–23°S) during the extreme mortality event of 2008 were analysed. The isotopic profiles of this species and their prey in a wintering area are presented to identify trophic relationships and to evaluate whether the prey species recovered in the stomach contents were assimilated. The prey groups recorded were molluscs (cephalopods and gastropods), teleost fish, and, to a lesser extent, crustaceans (decapods and isopods). Cephalopods were the most representative prey, and Argonauta nodosa was the most abundant species. Plant remains and solid waste were atypical items found in the stomach contents. The nitrogen isotope (δ15N) values found in this study confirm the higher trophic position of the penguins (14.5‰) relative to their prey (11.7–12.3‰). The carbon isotope (δ13C) measurements of all species are characteristic of marine coastal environments (−18.7 to −16.8‰). A Bayesian approach applied to stable isotope mixing models showed that cephalopods are assimilated to a greater extent than fish. However, the poor nutritional condition of specimens that reach the Brazilian coast, especially at the northern limit of migration (~21°S), indicates that prey ingestion is not sufficient for the maintenance of body weight.