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

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Abstract
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 ( ). We used the doubly labeled water method to measure active period field metabolic rates ( ) of verdins in their natural habitat, and compared our results to laboratory measurements of daytime resting metabolism ( ) and time-budget (TB) estimates of . Active-period metabolic expenditures decreased with increasing , but the temperature dependence of difered from that of such that in the cold the two functions converged. TB estimates of (computed by a model assuming activity costs were additive) were higher than measured at low . These results support the hypothesis that proportional substitution of exercise thermogenesis partially accommodates thermoregulatory requirements of small, free-ranging birds.