<p>The bursa of Fabricius as a unique trace immune organ in birds plays an important role in immune defense and provides a necessary microenvironment for the immune B cells development. Moreover, the body condition of the birds is closely related to the bursa’s development and degeneration. This study investigated the developmental and degenerative of the bursa of Fabricius in tree sparrows (<i>Passer montanus</i>) through analyses of body condition, bursa of Fabricius coefficient, structural changes, and B cell content. The results showed that the bursa of Fabricius in tree sparrows reached its developmental peak at 1 month of age and completely degenerated at approximately 5 months of age. During this process, individuals with better body condition exhibited faster bursal development and degeneration cycles, which contributed to the optimization of the immune system. Histological observations revealed morphological and structural changes across developmental stages, with B cells primarily distributed in the cortex and exhibiting proliferation, differentiation, and maturation. Using LASSO regression model, we analyzed key predictors, including age, bursa mass, and lymphoid follicle characteristics, that influence bursa development, degeneration and bursal B cells, with predictive models offering insights into the factors driving these processes. This study elucidated the developmental and degenerative cycles and immune functions fate of the bursa of Fabricius in the widely distributed altricial avian species, providing a foundation for understanding immune system regulation and its implications for avian health and disease prevention.</p>