Parental Care Modulates Early-Life Oral Microbiome Assembly and Immune Ontogeny in Wild Birds

A young bird meets its first microbes before its immune system is finished being built. By early-life microbiome dynamics we mean the changes in which microbes arrive, persist, and are lost over the first days and weeks of life. When we understand those dynamics, we can tell how much of a wild bird's microbiome its parents hand to it and how much its own maturing mucosa selects. We can then ask how that first community trains the immune system that will later regulate it. Developing birds offer a model for this work because rapid shifts along avian host developmental trajectories continually reshape the physiological and biochemical landscape of the oral mucosa. In turn, these dynamic transitions modulate local selective pressures, dictating which microbial lineages persist, interact, and establish long-term residence. The avian oral cavity thus serves as a tractable experimental interface for studying host-symbiont dynamics within a coupled holobiont framework. Tracking primary microbial colonizers, such as bacteria and archaea, alongside key host developmental milestones, such as the onset of endothermy in altricial nestlings, which alters physiological profiles like temperature and pH, enables us to directly observe ontogenetic microbial succession and gain insight into the ecological mechanisms governing early-life community assembly. Evaluating the holobiont as a unified unit of selection provides a mechanistic framework for understanding ontogenetic succession-one that can be tested against alternative assembly models such as host-genetic control, diet-driven replacement, and stochastic colonization. Although current evidence confirms that the oral microbiome serves as a distinct ecological record of environmental exposure and anatomical site, its ability to actively shape host immune phenotypes, stress responses, or overall fitness demands rigorous experimental testing. The nestling oral cavity provides a highly accessible surface for longitudinal research, where the "host clock" of mucosal immune maturation and the "microbial clock" of community assembly run in tandem. Validating or refuting this framework will require within-individual longitudinal sampling alongside targeted manipulations of mucosal immunity (e.g., IgA), advancing the field toward a mechanistic grasp of how early environmental exposures establish lasting biological organization.