Biogeographic patterns of animals are shaped by biotic interactions, such as competition, and by abiotic factors, such as climate. Mountain Chickadees (Parus gambeli) and Juniper Titmice (Parus ridgway) are permanent residents of regions of western North America and are ecologically similar, but have different northern range limits. l measured several physiological variables, including basal metabolic rate (BMR), peak metabolic rate (PMR = maximal thermogenic capacity), metabolic response to varying environmental temperature (MRT), evaporative water loss (EWL), and daily energy expenditure (DEE) for summer-and winter-acclimatized Mountain Chickadees and Juniper Titmice to determine if seasonal and interspecific variation in cold tolerance and thermogenic ability shape the northern range distribution of these two species. In addition, I examined the ecological consequences of nocturnal hypothermia and cavity roosting in seasonally acclimatized Mountain Chickadees and Juniper Titmice. Winter birds tolerated colder test temperatures than summer birds for both species This improved cold tolerance was associated with a significant increase in PMR in winter chickadees (27.1 %) and titmice (114%) compared to summer. BMR was significantly higher in winter birds (16.0%) compared to summer birds for both species. BMR and PMR were significantly higher for chickadees compared to titmice in both summer and winter. Winter chickadees were able to withstand colder test temperatures than winter titmice. The Mountain Chickadee's lower critical temperature is lower than the Juniper Titmouse's in summer and in winter. The Mountain Chickadee's upper critical temperature is also lower than the Juniper Titmouse's and chickadees also had significantly higher evaporative water loss rates compared to titmice. Seasonal acclimatization in Mountain Chickadees involves insulatory as well as metabolic changes. For Juniper Titmice winter acclimatization appears to be primarily a metabolic process. The laboratory metabolism data for activity costs associated with DEE revealed that foraging energy requirements were not significantly higher than alert perching energy requirements. DEE was significantly higher (P<0.05) in winter-acclimatized chickadees and titmice compared to their summer counterparts. The marked increase in calculated DEE in winter birds compared to summer contrasts a pattern of increased DEE in the breeding season for several avian species. The data from this study indicate that the northern range limit of small birds can be limited by energetic and water balance demands.