Heat Produced as a By-Product of Foraging Activity Contributes to Thermoregulation by Verdins, Auriparus flaviceps

To test the hypothesis that heat produced as a by-product of physical activity substitutes for avian thermoregulatory requirements, we measured daytime CO₂ production of a very active small passerine, the verdin (Auriparus flaviceps), at various standard operative temperatures (Tes). We used the doubly labeled water method to measure active period field metabolic rates (FMRα) of verdins in their natural habitat, and compared our results to laboratory measurements of daytime resting metabolism (RMRα) and time-budget (TB) estimates of FMRα. Active-period metabolic expenditures decreased with increasing Tes, but the temperature dependence of FMRα difered from that of FMRα such that in the cold the two functions converged. TB estimates of FMRα (computed by a model assuming activity costs were additive) were higher than measured FMRα at low Tes. These results support the hypothesis that proportional substitution of exercise thermogenesis partially accommodates thermoregulatory requirements of small, free-ranging birds.