Thermoregulation and Habitat Preference in Mountain Chickadees and Juniper Titmice

The Mountain Chickadee (Poecile gambeli) and the Juniper Titmouse (Baeolophus ridgwayi) are closely related, ecologically similar passerines sympatric in portions of their range. However, Mountain Chickadees prefer higher altitude, cooler habitats than Juniper Titmice. We measured oxygen consumption, evaporative water loss, body temperature, and thermal conductance on seasonally acclimatized individuals to determine if thermoregulatory differences correlate with habitat preference. The Mountain Chickadee’s lower critical temperature was 4.28C lower than the Juniper Titmouse’s in summer and 2.48C lower in winter. Thermal conductance decreased significantly in winter relative to summer in Mountain Chickadees but not in Juniper Titmice. The Mountain Chickadee’s upper critical temperature was 4.28C lower than the Juniper Titmouse’s in summer. Also in summer, Mountain Chickadees had significantly higher body temperature above the upper critical temperature than Juniper Titmice, indicating less heat tolerance. The overall metabolic response to temperature in these two species suggests that physiology plays a role in maintaining their habitat segregation.