In birds, acclimation and acclimatization to temperature are associated with changes in basal (BMR), summit (Msum) and maximal (MMR) metabolic rates but little is known about the rate at which species adjust their phenotype to short-term temperature variations. Our aims were (1) to determine the pattern of metabolic adjustments following a rapid temperature change, (2) to determine whether performance varies at similar rates during exposure to warm or cold environments, and (3) to determine if BMR, Msum and MMR change at comparable rates during thermal acclimation. We measured these parameters in white-throated sparrows (Zonotrichia albicollis), black-capped chickadees (Poecile atricapillus), and snow buntings (Plectrophenax nivalis) after acclimation to 10 °C (day 0) and on the 4th and 8th days of acclimation to either -5 or 28 °C. Birds changed their metabolic phenotype within 8 days with patterns differing among species. Sparrows expressed the expected metabolic increases in the cold and decreases at thermoneutrality while performance in chickadees and buntings was not influenced by temperature but changed over time with inverse patterns. Our results suggest that BMR varies at comparable rates in warm and cold environments but changes faster than Msum and MMR, likely due to limitations in the rate of change in organ size and function. They also suggest that maximal metabolic capacity is lost faster in a warm environment than it is gained in a cold environment. With the expected increase in temperature stochasticity at northern latitudes, a loss of thermogenic capacity during warm winter days could, therefore, be detrimental if birds are slow to readjust their phenotype with the return of cold days.

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