Acclimatization to winter conditions is an essential prerequisite for survival of small passerines. Seasonal changes in a bird's physiology and behavior are considered to be part of an adaptive strategy for survival and reproductive success. Changes in photoperiod, ambient temperature and food availability trigger seasonal acclimatization in physiology and behavior of many birds. In the present study, seasonal adjustments in several physiological, hormonal, and biochemical markers were examined in wild-captured Chinese bulbuls ( Pycnonotus sinensis
) from the Zhejiang Province in China. Oxygen consumption was measured using the closed-circuit respirometer containing 3.6 L animal chambers. State-4 respiration in liver and muscle mitochondria was measured at 30°C with a Clark electrode. The activities of cytochrome C oxidase (COX) in liver and muscle were measured polarographically at 30°C using a Clark electrode. The protein content of mitochondria was determined by the Folin phenol method, with bovine serum albumin as standard. In winter sparrows had higher body mass and basal metabolic rate (BMR). The contents of mitochondrial protein in liver, and state-4 respiration and COX activity in liver and muscle increased significantly in winter. Circulating level of serum triiodothyronine (T3) was significantly higher in winter than in summer. Together, these data suggest that Chinese bulbuls mainly coped with cold by enhancing thermogenic capacities through increased activity of respiratory enzymes activities. The results support the view that prominent winter increases in BMR are manifestations of winter acclimatization in Chinese bulbuls and that seasonal variation in metabolism in bulbuls is similar to that in other small wintering birds.