Physiological adaptations for overwintering by the black-capped chickadee (Parus atricapillus) in interior Alaska (64 degrees North latitude)

Abstract

Winter in interior Alaska (64$\sp\circ$N) is characterized by short photoperiod (LD 5:19) and chronic subfreezing temperatures. These conditions present a physiological challenge to overwintering animals. This challenge increases as body size decreases. In small animals, higher surface to volume ratio and concomitant higher mass-specific metabolic rate (compared to larger animals) vastly increase energetic demand. This thesis focuses on the physiological adaptations of the smallest (12 g) overwintering bird in interior Alaska, the Black-capped Chickadee (Parus atricapillus). To determine if seasonal acclimatization of Black-capped Chickadees at high latitude differs from that of conspecifics at lower latitudes, standard metabolic rates (SMR), metabolic response to low temperature (${-}30\sp\circ$C), use of nocturnal hypothermia, body mass, fat reserves, and conductance were measured over two winters and one summer in three groups of seasonally acclimatized captive birds. Body mass and conductance did not vary with season, but furcular fat levels were higher in winter. Birds used nocturnal hypothermia when exposed to ${-}30\sp\circ$C in summer or winter. Although SMR did not vary seasonally, winter SMRs differed between the two winters studied. Nocturnal hypothermia in summer and decreased SMR during winter have not been reported in conspecifics at lower latitudes. Lipid reserves play an important role in fueling the energy demands of overwintering birds. Black-capped Chickadees exhibit a daily and seasonal cycle of fattening. At high latitudes (64$\sp\circ$N), fat to fuel an 18 hour nocturnal fast is deposited during 6 hours of daylight available for foraging. Daily fattening rates are highest in December when ambient conditions are most limiting. Activity of the fat deposition-promoting enzyme, adipose lipoprotein lipase (ALPL), was measured in furcular fat samples from seasonally acclimatized captive birds to determine its role in daily and seasonal fattening cycles. ALPL activity levels were 20-30 times higher than those previously reported for passerines. Seasonal variation in enzyme activity positively correlated with changes in fattening rates from December to April. Alaskan birds displayed the highest level of activity when compared to conspecifics at lower latitudes. All subpopulations measured exhibited activity higher than previously reported for small birds.