Coping with the cold: black-capped chickadees (Poecile atricapillus) use rapid facultative hypothermia to reduce fat loss overnight.

Heat loss due to a large surface-area-to-volume ratio makes surviving periods of cold temperatures a challenge for small endothermic animals. Many temperate, non-migratory birds survive cold winter nights by increasing heat production through shivering, maintaining their body temperature at the cost of the energy provided by fat stores. An additional strategy may be the use of facultative hypothermia, allowing body temperature to decrease overnight to conserve energy rather than maintaining normothermic body temperature. In this study, we examined the relationship between overnight body temperature and fat loss under a cold temperature challenge in black-capped chickadees (Poecile atricapillus). To investigate this, we brought 10 wild black-capped chickadees into captivity and used quantitative magnetic resonance analysis and temperature sensitive radio transmitters to compare changes within-individual overnight fat loss and body temperature at air temperature of 20 °C versus 3 °C. Most of the black-capped chickadees showed both reduced body temperature and greater fat loss at 3 °C compared to 20 °C. Furthermore, individuals exhibiting greater decreases in body temperature showed lower overnight fat loss. Interestingly, some individuals did not drop their body temperature across the air temperature conditions. These results suggest individual variation in use of facultative hypothermia to reduce fat loss overnight and provide valuable insights into the metabolic strategies small, non-migratory birds use to survive cold nights.