The aim of this programme was to study the physiological and behavioural adaptations to the incubation fast in the female eider. This leads to study fundamental questions about three complementary field researches described below. 1. Evolutionary and ecological approaches: energetic costs of reproduction during incubation In long-lived birds as Eider, there must be trade-offs between the energy allocated in growth and in reproduction. Therefore, individuals develop different reproductive strategies in relation with biotic and non biotic factors to maximize their fitness. Among factors tested, we will first measure the effects of animal density on female reproductive success. Additionally, we will measure, thanks to genetic tests, 1) the characteristics of eider populations (dispertion) by comparing birds originating from several islands and several locations on the same island, 2) the frequency of intra-specific nest parasitism and 3) extra-pair copulations to link these events with female behavioural decisions. To link reproductive effort with female immunocompetence, we will then perform PHA (phytohaemagglutinine) skin tests at different stages of the incubation period. Finally, we will perform clutch reductions at different stages of the incubation period in order to highlight decision rules controlling nest desertion in females. 2. Physiological and ecological approaches: parental investment in reproduction We will also focus on the implication of prolactin and corticosterone in the control of parental decision at the hatching stage. Implantation of exogenous hormones will be done on nesting birds to evaluate the respective role of these two hormones in the control of parental decisions in eiders. Parental investment in incubation can be regulated by the reproductive value of the clutch size. To further understand the mechanism underlying nest desertion, we will measure the induced-changes in prolactin and corticosterone concentrations after clutch size manipulation overall the incubating period. 3. Physiological approach: regulation of body fuel utilization during fasting The aim will be to study the mechanisms of the regulation of body fuel utilization and energy expenditure during fasting. For this purpose, the ability of eider duck to withstand long periods of starvation will be studied by measuring the variations in plasma of major substrate concentrations (as index of lipid or protein breakdown) and hormones (e.g., leptin, glucagon, corticosterone, T3, ...). The study of duck’s adaptation to extended fasts occurring at specific stages of their life might help to understand important underlying mechanisms, such as reduction in energy expenditure, long-term regulation of body fat storage and mobilization, as well as long-term control of food intake.