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iii Differences in organismal life-history characteristics are often related to variation in an organisms ’ energy expenditure. For example, tropical bird species show a significantly slower “pace of life ” in terms of increased longevity, reduced fecundity and faster maturation when compared with temperate species and this slower pace is correlated with a reduced basal metabolic rate (BMR). Differences in BMR could have a genetic basis yet the mechanisms governing energy expenditure remain unclear. Genes encoding proteins involved in crucial energetic pathways present logical candidates to explore the role of selection as a cause of energetic differences between species. The distribution of certain bird families in both tropical and temperate environments offers a unique opportunity to study the molecular evolution of these physiological adaptations through paired comparisons of mitochondrial DNA evolution among confamilial species. Here, I explored the role of evolution of mitochondrial proteins as a genetic cause for variation in mass-organismal energy expenditure by examining the role of selection o