Abstract
Animals often create unique microenvironments during reproduction, which may influence offspring health and development by altering the microbiota within these environments. Recently, nest microbiota has gained increasing recognition as integral components of early-life environments. Therefore, using the Japanese Tit (Parus minor) as a model species, this study investigates whether nest construction modifies nest microbiota and whether such changes correlate with reproductive performance. During the post-nesting stage, the results showed that both bacterial diversity and fungal richness in the nest significantly increased, with beneficial bacteria being notably enriched and potential pathogens decreased. Co-occurrence network analysis revealed intensified microbial competition in the post-nesting stage. Community assembly analyses indicated that bacterial communities in the post-nesting stage were primarily governed by deterministic processes, whereas fungal communities were mainly constrained by dispersal limitation. Source tracking revealed that nesting material composed of moss serves as one of the key sources of microbial community composition within the nest. Notably, most beneficial microbes were positively correlated with clutch size, incubation period, and number of hatchlings, while certain pathogens were negatively correlated. Overall, these findings demonstrate that cavity-nesting birds can reshape the nest microbiota through nesting material, promoting beneficial symbionts while suppressing potential pathogens. Such host-mediated regulation of early-stage microbiomes may represent an important adaptive strategy to cope with reproductive pressures.