Abstract: Temperature and photoperiod are proximate environmental factors that have an important influence on the morphological, physiological and behavioral adjustments animals performance du-ring seasonal acclimatization. In this study, the effects of temperature and photoperiod on phenotypic flexibility in body mass, energy budget and digestive tract morphology in the Chinese bulbul (Pycnonotus sinensis) were examined, and the relationship between energy budget and digestive tract morphology was analyzed. Twelve male and sixteen female Chinese bulbuls were randomly assigned into 4 experimental groups so that each group was comprised of three males and four females. The groups were: 1) a warm and long photoperiod (30 ℃, 16 light: 8 dark) group, 2) a warm and short photoperiod (30 ℃, 8 light: 16 dark) group, 3) a cold and long photoperiod (10 ℃, 16 light:8 dark) group, and 4) a cold and short photoperiod (10 ℃, 8 light: 16 dark) group. Each group was acclimated to its respective temperature and photoperiod for 4 weeks. Birds in the cold temperature and short photoperiod group underwent a significant increase in body mass, gross energy intake (GEI) and digestible energy intake (DEI) compared to the other three groups, and there was a significant interaction between temperature and photoperiod on gross energy intake and diges-tible energy intake. The mass of the stomach, small intestine, rectum, and total digestive tract, all increased significantly in cold temperature treatment groups compared to those acclimated to a relatively warm temperature. There was a significant, positive correlation between GEI and DEI resi-duals and those of the length and dry mass of the small intestine and total digestive tract. These results suggested that the Chinese bulbul met the increased energy demands of winter (colder tempe-ratures and reduced foraging time due to shorter day-length) by increasing its body mass, digestible energy intake and digestive tract size.