Survival over growth: Accelerated bursa of Fabricius development enhances immune resilience in heavy metal-exposed tree sparrows.

The bursa of Fabricius (BF), a regressive lymphoid organ unique to avian species, plays a pivotal role in early immune defense post-hatching: it not only mediates innate immune responses but also provides a microenvironment necessary for B-cell maturation, thereby playing an indispensable role in the development and functional maturation of the avian adaptive immune system. Life history theory predicts energy allocation trade-offs in response to environmental challenges, which often suppresses costly processes like immunity to prioritize survival. This study investigated effects of long-term environmental heavy metal pollution on these trade-offs and BF development in the tree sparrow (Passer montanus). Contrary to the paradigm of stress-induced immunosuppression, we found that despite significant impairments in body condition and growth rate, nestlings from a polluted site exhibited enhanced BF development. This was demonstrated through three key phenotypic adaptations: (1) an increased BF coefficient, (2) accelerated follicular development and histological maturation, and (3) increased B-lymphocyte density and migration to the spleen. Nestlings from a polluted site also displayed enhanced innate immunity and antioxidant defenses. An Integrated Biological Response (IBR) model suggested a strategic energy reallocation where investment in growth was suppressed to prioritize immune organ development and function. These findings provide a novel perspective on adaptive life-history strategies, demonstrating that birds can prioritize immune resilience to persist in contaminated environments at a cost to somatic development.