A large number of physiological acclimation studies assume that Xexibility in a certain trait is both adaptive and functionally important for organisms in their natural environment; however, it is not clear how an organism’s capacity for temperature acclimation translates to the seasonal acclimatization that these organisms must accomplish. To elucidate this relationship, we measured BMR and TEWL rates in both Weld-acclimatized and laboratoryacclimated adult rufous-collared sparrows (Zonotrichia capensis). Measurements in Weld-acclimatized birds were taken during the winter and summer seasons; in the laboratory- acclimated birds, we took our measurements following 4 weeks at either 15 or 30°C. Although BMR and TEWL rates did not diVer between winter and summer in the Weldacclimatized birds, laboratory-acclimated birds exposed to 15°C exhibited both a higher BMR and TEWL rate when compared to the birds acclimated to 30°C and the Weldacclimatized birds. Because organ masses seem to be similar between Weld and cold-acclimated birds whereas BMR is higher in cold-acclimated birds, the variability in BMR cannot be explained completely by adjustments in organ masses. Our Wndings suggest that, although rufous-collared sparrows can exhibit thermal acclimation of physiological traits, sparrows do not use this capacity to cope with minor to moderate Xuctuations in environmental conditions. Our data support the hypothesis that physiological Xexibility in energetic traits is a common feature of avian metabolism.
