The results show that the effective efficiency of mitochondrial oxidative phosphorylation adjusted to the nutritional status of winter king penguin chicks. This may be summarized as follows: (i) mitochondria from fasted chicks minimized the cost of ATP synthesis by decreasing the needs for energy substrates. (ii) This plasticity would be more pronounced when birds become hypothermic during winter. (iii) Mitochondria from re-fed chicks were in turn more ‘thermogenic’, oxidizing more nutrients and thus producing more heat to synthesize the same amount of ATP as fasted chicks. This phenotypic flexibility of mitochondrial energy transduction processes allows king penguin chicks to adjust the cost of their metabolic performance to cope with stochastic parental food provision during the austral winter.