Abstract: The annual moult creates the highest physiological stress during a penguin’s breeding cycle and is preceded by a period of hyperphagia at sea. Although crucial to individual survival, foraging strategies before moult have been little investigated in keystone marine consumers in the Southern Ocean. The Macaroni Penguin Eudyptes chrysolophus demonstrates how individuals may adjust their foraging strategies during this period in line with
constraints such as potential intraspecific competition between localities, foraging ability between dimorphic sexes and timing at sea between breeding and non-breeding population components. We recorded pre-moult behaviour at sea for 22 Macaroni Penguins from Crozet and Kerguelen Islands (southern Indian Ocean) during 2009 and 2011, using light-based geolocation and stable isotope analysis. Penguins were distributed in population-specific oceanic areas with similar surface temperatures (3.5 °C) south of the archipelagos, where they foraged at comparable trophic levels based on stable isotopes of their blood. Bayesian ‘broken stick’ modelling with concurrent analysis of seawater temperature records from the animal-borne devices showed that within each population, females remained 6 days longer than males in the colder waters before heading back towards their colonies. Finally, 17 other non-breeding individuals that moulted earlier had a higher mean blood d15N value than did post-breeding birds, meaning that early moulters probably fed more on fish than did late moulters. Our findings of such adjustments in foraging strategies developed across locality, sex and breeding status help understanding of the species’ contrasted pre-moult biology across its range and its ecology in the non-breeding period.