The necessity of maintaining energy balance is one of the major factors influencing the abundance and distribution of small birds,and many morphological,physiological and behavioral adaptations are related to energy utilization.Basal metabolic rate(BMR) is the rate of energy transformation in a rest,awake and postabsorptive state in the absence of thermal stress,and is the minimum metabolic rate of animals maintaining normally physiological function.Theoretically,BMR limits might be set by central organs(primarily visceral organs such as the pulmonary,cardiovascular,digestive or excretory systems) or by peripheral organs(primarily skeletal muscles) that support peripheral effectors.These ideas represent the ’central limitation hypothesis’ and the ’peripheral limitation hypothesis,’ respectively.In this study,we measured seasonal changes in BMR and organ mass,and evaluated the relationship between BMR,organ mass in Chinese Bulbul(Pycnonotus sinensis).The results were used to test the ’energy demand’ hypothesis for adjustments in metabolic rate.The energy demand hypothesis supposed that birds adjust the size of their internal organs relative to food intake,a correlate of energy demand.Winter and autumn bulbuls demonstrated significantly higher BMR than their spring and summer counterparts.Winter and autumn birds also had higher liver,heart,gizzard,small intestine,kidneys,total digestive tract and muscle than spring and summer birds.These data support the hypothesis that prominent winter increases in BMR are components of winter acclimatization in Chinese Bulbuls and that seasonal changes in metabolism in bulbuls are similar to those for other small wintering birds.Meanwhile,these data test the hypothesis that central and peripheral organs have a significant positive relationship with BMR.The relationship between BMR and internal organs could be due to either(or both) increased energy demand or seasonal shifts in diet.