Energy is by definition the resource that is allocated to all active processes within an organism, and thus involved in all life-history trade-offs. Traditionally energy flows have been quantified by measuring the respiration of whole organisms. Yet, the whole-organism respiration is ultimately a function of energy flow at the subcellular level, in which the mitochondria play a principal role. Through oxidative phosphorylation mitochondria produce more than 90% of the energy required by eukaryotic cells. The efficiency with which mitochondria function to produce such energy is increasingly linked to variation in animal performance, thereby constituting a promising research target for explaining variation in individual quality within the framework of life-history theory. 

Birds are ideal in this respect, because their red blood cells contain functional mitochondria. This allows repeated sampling of individuals in a minimally-invasive way. While tissue sampling is often very invasive, collecting a small blood sample can be done routinely with little harm to the individual. By sampling adults and chicks in both relatively long-lived wild common terns and relatively short-lived captive Japanese quail, and the use of a state-of-the-art high-resolution respirometry measurement system, we can quantify causes and life-history consequences of variation in mitochondrial performance within and among individuals. In addition, experimental manipulation of environmental variables in the captive Japanese quail allows us to establish causal effects on mitochondrial performance.