Morphological and physiological correlates of among- individual variation in basal metabolic rate in two passerine birds.

Basal metabolic rate (BMR) has been shown to be a highly phenotypic flexibility trait within species. A significant proportion of an individual's energy budget is accounted for by BMR, hence among-individual variation in this trait may affect other energetic processes, as well as fitness. In this study, we measured BMR, organ mass, mitochondrial respiration capacities and cytochrome c oxidase (COX) activities in muscle and liver and circulating levels of plasma triiodothyronine (T) in Chinese bulbuls (Pycnonotus sinensis) and Eurasian tree sparrows (Passer montanus).