Migration-driven microbial adaptation and ecological spillover in birds.

Migratory birds perform one of the most physiologically demanding feats in the animal kingdom, rapidly accumulating fat reserves and enduring extreme environmental and immunological stress. Central to their survival is the gut microbiota, a diverse assemblage of microorganisms that contributes to energy harvesting, immune modulation, and host adaptation. As birds traverse varied landscapes and feed on diverse diets, their gut microbial communities undergo marked compositional and functional shifts. These changes can optimize nutrient absorption and immune preparedness, but they may also lead to dysbiosis under conditions of stress or pathogen exposure, potentially impairing migratory performance. Importantly, migratory birds also act as mobile reservoirs of zoonotic pathogens and antimicrobial resistance genes. Stopover sites, critical refueling points along migratory routes, serve as hubs for microbial exchange between wild birds, domestic animals, and human-altered environments, thereby amplifying spillover risks. We highlight current gaps in understanding the forces that remodel the gut microbiota and mechanistic links between microbiota dynamics and migratory performance, and propose integrative research strategies involving longitudinal sampling, meta-omics, and controlled experiments. Ultimately, bird migration offers a powerful model for exploring host-microbe co-adaptation under extreme ecological pressures. Addressing these dynamics through a One Health framework is essential for biodiversity conservation, disease mitigation, and global health security.