SEASONAL ACCLIMATIZATION OF THERMOREGULATION IN THE BLACK-CAPPED CHICKADEE

Black-capped Chickadees (Parus atricapillus) show behavioral adaptations (food caching, cavity roosting) and can undergo regulated bouts of nocturnal hypothermia, both of which reduce costs associated with wintering in temperate latitudes. These adjustments could reduce the need for the seasonal metabolic adjustments found in other small passerines that must deal with severe winter cold. We have examined this possibility by determining the extent of seasonal variation evident in chickadees concerning standard metabolic rate, metabolic response to temperature, cold resistance, and maximal thermogenic capacity (Vo(2)(sum)). Regression equations relating metabolism to air temperature below thermoneutrality did not differ significantly in either slope or Y-intercept between summer and winter, and neither did minimum thermal conductance for normothermic birds vary seasonally. Seasonal constancy in these parameters demonstrates the minor role that seasonal changes in insulation play in acclimatization in chickadees. However, winter birds tolerated cold stress far better than summer birds. This improved cold tolerance was associated with a significant increase in Vo(2)(sum) (36%) in winter relative to summer. Standard metabolic rates (SMR) were also significantly increased in winter birds compared to summer birds. Thus, Black-capped Chickadees show seasonal metabolic acclimatization similar to, or greater than, other temperate wintering passerines in addition to behavioral adaptations and nocturnal hypothermia.