Abstract: Species distribution of animals is closely related to the metabolic thermogenesis. Basal metabolic rate (BMR) plays an important role in metabolic thermogenesis in endotherms. In this study, we examined the changes in body mass, body temperature and metabolic rate (MR) over a range of ambient temperatures (15, 20, 22, 24, 26, 28, 30, 32, 34, 36, 38 and 40 ℃, respectively) in black-throated bushtits (Aegithalos concinnus) acclimated to cold (10 ℃) or normal (23 ℃) ambient temperature conditions for four weeks, and then calculated thermal conductance. The MR and body temperature were measured by open oxygen analyzer and a lubricated thermocouple inserted into the cloaca, respectively. Results showed that body mass and BMR of the 10 ℃ group were 6.3±0.1 g and 23.42±0.28 mL O²·h^-1, respectively, while the body mass and BMR of the 23 ℃ group were 5.9±0.1 g and 18.80±0.28 mL O²·h^-1, respectively. The thermal neutral zones (TNZ) of the 10 ℃ group and the 23 ℃ group were 26-38 ℃ and 28-34 ℃, respectively. Below the lower critical temperature, MR increased linearly with decreasing ambient temperatures (T_a), and the fitting equations of MR for the 10 ℃ group and the 23 ℃ group were MR=52.64-1.06T_a and MR=47.26-1.02T_a, respectively. Taken together, the black-throated bushtits acclimated at 10 ℃ underwent a significant increase in body mass and BMR. Furthermore, the cold-acclimated black-throated bushtits showed a lower critical temperature and a wider TNZ compared with the birds kept at 23 ℃. The plasticity of metabolism and thermogenesis benefited the birds in adapting to variations in ambient temperature to acquire a maximum fitness.

Key words: Aegithalos concinnus, ambient temperature, lower critical temperature, metabolic thermogenesis, thermal neutral zone