Reassessing the definition of basal metabolic rate: Circadian considerations in avian studies

The importance of daily fluctuations in avian basal metabolic rate (BMR) was introduced in the classic paper by Aschoff and Pohl (1970) characterizing circadian differences. Since that time, there has been a shift in the BMR literature to accept only values measured in the ρ- (resting) phase of the day as true BMR. We argue that α- (active) and ρ-phases both represent BMR, analogous to the plasticity of BMR seen in seasonal variations, or associated with changes in body composition and restrictions in diet. Furthermore, we demonstrate that circadian differences in BMR are not even present in most polar birds and are sometimes absent in a variety of non-passerines. We argue for the ecological value of a 24 h BMR in order to compare with daily energy expenditure (field metabolic rate, FMR), which integrates entire days. We make recommendations for the reporting of BMR with sufficient information so readers will know time, season, etc. We also suggest that measures of BMR can be accepted during reproduction in birds, though with some caution.