Climate warming is predicted to have pronounced effects on biodiversity, particularly the biogeography of organisms, such that species’ range distributions should contract at warmer and expand at cooler range margins, via influences on population-demographic processes, resulting in overall higher latitudinal or elevational displacement of individuals. Penguins (Genus Pygoscelis) of the Palmer Archipelago, west Antarctic peninsula, are currently demonstrating expected responses to rapid regional climate warming; a breeding population decline and range contraction is occurring in the sea-ice dependent Adélie penguin (P. adeliae), a true Antarctic species, coincident with breeding population increases and range expansions by sea-ice independent species such as the gentoo (P. papua) and chinstrap (P. antarctica) penguin, both generally considered sub-Antarctic in distribution. The proximate mechanisms by which climate warming and associated sea-ice decline drive such marked responses by top-level marine predator populations remain elusive. Here, we examine relationships between nutritional and physiological stress parameters and breeding performance traits across all three species of Pygoscelis to better understand mechanistic links between variation in diet, as inferred from naturally occurring stable carbon and nitrogen isotope signatures of adults and offspring, body condition, physiological response indicated by circulating corticosterone, oxidative stress, and immunoglobulin levels, and adult reproductive performance based on decisions concerning timing of egg-laying, egg size, offspring sex ratio, and sex-specific offspring quality. The present study will lend insight to the proximate mechanisms by which climate-induced changes in food-web structure might contribute, either negatively or positively, to individual fitness and ultimately population dynamics.