Age-related changes in the gut microbiota of a long-lived seabird suggest divergence from mammalian models.

Gut microbiota plays key roles in shaping host development, immunity and physiology. The assembly of vertebrate gut microbiota during early life follows relatively predictable trajectories across several mammalian species, largely driven by maternal transmission routes. However, birds lack these transmission mechanisms owing to their oviparity, potentially leading to different age-related microbiota patterns. Here, we investigate gut microbiota variation in Atlantic puffin (Fratercula arctica) chicks and adults across six British populations spanning distinct marine regions using 16S rRNA amplicon sequencing. We find that chick microbiotas show strong population-level structuring that disappears in adults, with spatial patterns particularly pronounced in anaerobic versus aerotolerant components of the microbiota. Puffin microbiotas become increasingly individualized and aerotolerant with age, while taxonomic richness remains stable-patterns that contrast with the convergent, richness-increasing trajectories observed in several mammalian species. Our results suggest that environmental exposure is a key driver of early-life microbiota assembly in birds, highlighting the value of avian systems for understanding vertebrate microbiota development beyond mammals.