All animals rely on energy in order to grow, reproduce and thrive. The conversion of nutrients into energy happens in mitochondria, which are crucial components of the cells. The efficiency with which mitochondria produce energy varies within species and is increasingly thought to play a fundamental role in explaining individual heterogeneity in growth, reproduction and survival. Indeed, mitochondrial metabolism and efficiency can vary through phenotypic plasticity in response to environmental factors such as temperature, food availability or any stressful stimulus. However, despite the importance of environmental determinants in metabolism, we still lack key information about wild species within their natural habitat. As a result, physiological capacities and potential responses to environmental changes are underappreciated, especially in birds. With this doctoral project, I explore how red blood cell mitochondrial metabolism and its efficiency vary in response to different environmental challenges in two distinct wild bird species (the Great tit and the King penguin) within their ecosystem and at various life stages (both during growth and adulthood). Here, I investigate the impact of early-life environmental adversity, nutritive stress and maternal effects on mitochondrial metabolism, and, in turn, its impact on individual growth and survival, in order to provide much-needed information for understanding how mechanistic constraints at a cellular level may shape the ecology and adaptability in two different avian species in natura.