Seabird foraging behaviour varies through time according to factors that may be related to life cycle stage and/or habitat. However, no study has previously investigated variation in these different factors and how they impact foraging behaviour in a single species simultaneously. The aim of this thesis was to distinguish the fixed part of foraging behaviour from its different degrees of plasticity in a pantropical seabird, the Red-footed booby (Sula sula). Adults and juveniles from different colonies were equipped with GPS tags to track their foraging trips according to several intrinsic (age and breeding stage) and extrinsic factors (physical forcing, primary productivity and competition for resources). Adult foraging behaviour varied significantly within the same colony according to breeding stage. Tracks were shorter during brooding, which is known to be a particularly energy-constraining time period, and longer during incubation and fledging. Multi-year monitoring at one of the colonies revealed flexibility in behaviour under adverse environmental conditions. Longer tracks were observed during breeding seasons with decreased productivity and eddy activity. High plasticity in foraging behaviour according to habitat was observed across the range of the species. The length and range of adult trips varied considerably among colonies, ranging from strictly diurnal to longer trips that included several nights spent at sea. Red-footed boobies did not appear to target particularly productive areas, and intra- and interspecific competition seemed to explain partly the differences observed between colonies. On the other hand, all the individuals showed similar trip structuring and increased searching effort by adopting area-restricted search (ARS) behaviours. The behaviour of juveniles, still fed by their parents after fledging during a long transition period, was described for the first time. Over time, juveniles increased trip ranges, which were considerably lower than that of adults. Associations at sea with other juveniles were frequently identified. This long learning period seems to be an adaptation allowing the gradual acquisition of the skills necessary for the search and capture of prey. While the behavioural plasticity of individuals tends to cushion the effects of unfavourable conditions at the population scale, this flexibility is still limited. In the current context of global change, assessment of behavioural plasticity is necessary in order to better predict the consequences of these changes on populations.

Le comportement de recherche alimentaire varie tout au long de la vie des oiseaux marins selon divers facteurs qui peuvent ĂŞtre liĂ©s au cycle de vie ou Ă  l’habitat. Cependant, aucune Ă©tude n’avait auparavant abordĂ© ces diffĂ©rents facteurs de variation chez une seule et mĂŞme espèce. L’objectif de cette thèse Ă©tait de distinguer la part fixe du comportement de recherche alimentaire de ses diffĂ©rents degrĂ©s de plasticitĂ© chez un oiseau marin pantropical, le fou Ă  pieds rouges (Sula sula). Pour cela, des adultes et des juvĂ©niles issus de diffĂ©rentes colonies ont Ă©tĂ© Ă©quipĂ©s de balises GPS afin d’étudier leurs trajets selon diffĂ©rents facteurs de variation intrinsèques (âge et stade de reproduction) et extrinsèques (forçages physiques, productivitĂ© primaire, compĂ©tition pour la ressource). Le comportement de recherche alimentaire des adultes variait de façon importante au sein d’une mĂŞme colonie selon le stade de reproduction. Les trajets Ă©taient plus courts pendant l’Ă©levage du poussin, connu pour ĂŞtre particulièrement contraignant d’un point de vue Ă©nergĂ©tique, et plus longs pendant l’incubation et la pĂ©riode post-envol. Le suivi pluriannuel d’une des colonies a pu mettre en Ă©vidence la flexibilitĂ© du comportement lors de conditions environnementales dĂ©favorables. Des trajets plus longs Ă©taient observĂ©s lors de la saison de reproduction qui prĂ©sentait une diminution de la productivitĂ© et de l’activitĂ© tourbillonnaire locale. Une importante plasticitĂ© du comportement en fonction de l’habitat a pu ĂŞtre mise en Ă©vidence Ă  l’échelle de l’aire de rĂ©partition de l’espèce. La durĂ©e et le rayon de prospection des trajets des adultes variaient considĂ©rablement selon les colonies, allant de trajets strictement diurnes Ă  des trajets beaucoup plus longs qui incluaient plusieurs nuits passĂ©es en mer. Les fous Ă  pieds rouges ne semblaient pas cibler des zones particulièrement productives et la compĂ©tition intra- et interspĂ©cifique semblait expliquer en partie les diffĂ©rences observĂ©es entre les colonies. En revanche, tous les trajets possĂ©daient gĂ©nĂ©ralement une structure similaire et augmentaient leur effort de recherche alimentaire en adoptant un comportement de recherche en zone restreinte (ARS). Le comportement des juvĂ©niles, encore nourris par leurs parents après leur envol pendant une longue pĂ©riode de transition, a pu ĂŞtre dĂ©crit pour la première fois. Avec le temps, les juvĂ©niles augmentaient le rayon de prospection de leurs trajets, qui restait considĂ©rablement infĂ©rieur Ă  celui des adultes. Des associations en mer avec d’autres juvĂ©niles Ă©taient frĂ©quemment identifiĂ©es. Cette longue pĂ©riode d’apprentissage semble permettre l’acquisition progressive de compĂ©tences complexes nĂ©cessaires Ă  la recherche et Ă  la capture de leurs proies. Si la plasticitĂ© comportementale des individus tend gĂ©nĂ©ralement Ă  amortir les effets des conditions dĂ©favorables sur l’Ă©tat de santĂ© moyen d’une population, cette flexibilitĂ© est tout de mĂŞme limitĂ©e. Dans le contexte actuel de changement global, l’évaluation de la plasticitĂ© comportementale s’avère alors nĂ©cessaire afin de mieux prĂ©voir les consĂ©quences de ces changements sur les populations.