Background: Seasonal adjustments in body mass and energy budget are important for the survival of small birds in temperate zones. Seasonal changes in body mass, body temperature, gross energy intake (GEI), digestible energy intake (DEI), body fat content, as well as length and mass of the digestive tract, were measured in Chinese Bulbuls (Pycnonotus sinensis) caught in the wild at Wenzhou, China. Methods: Body mass was determined with a Sartorius balance. The caloric contents of the dried food and feces were then determined using a oxygen bomb calorimeter. Total fat was extracted from the dried carcasses by ether extraction in a Soxhlet apparatus. The digestive tract of each bird was measured and weighed, and was then dried to a constant mass. Results: Body mass showed a significant seasonal variation and was higher in spring and winter than in summer and autumn. Body fat was higher in winter than in other seasons. GEI and DEI were significantly higher in winter. The length and mass of the digestive tract were greatest in winter and the magnitude of both these parameters was positively correlated with body mass, GEI and DEI. Small passerines typically have higher daily energy expenditure in winter, necessitating increased food consumption. Conclusions: This general observation is consistent with the observed winter increase in gut volume and body mass in Chinese Bulbuls. These results suggest that Chinese Bulbuls adjust to winter conditions by increasing their body mass, body fat, GEI, DEI and digestive tract size.