Winter, the most challenging season for animals, is usually accompanied by extremely cold temperatures and limited food resources. Harsh winter conditions force birds to develop behavioral and physiological adaptations to reduce mortality. Birds might select for sexual segregation to reduce conflict between different social hierarchies at the population level. They can also adjust their body conditioning via fat reserve to balance the trade-off between starvation and predation at the individual level. Using mist-netting surveys and bird banding, we traced 61 orange-flanked bush-robin (Tarsiger cyanurus), an abundant and easily-observed long-distance migratory bird exhibiting delayed plumage maturation, to better understand the winter adaptation of these songbirds. We found that the number of 2yr+ adult males with bright-blue plumage was significantly lower than the number of males with olive-brown plumage. However, the sex-ratio was only slightly skewed to males with olive-brown plumage, suggesting that habitat-type influences sexual segregation. This robin tends to become heavier and store more fat over the course of winter, as they can fine-tune their fat reserve in response to changes in weather (including temperature, humidity, and snowfall) and food abundance. Interestingly, capturing the birds may also have a significant positive effect on their fat reserve. Overall, these results improved our understanding of the flexibility in adaptation of small passerine birds wintering in a subtropical forest and provided vision for considering the inevitable influence by ornithology field methods.