Early-life gut microbiota differentiation in sympatric wild raptors

The establishment of the gut microbiota during early life plays a crucial role in host physiology and development, yet remains poorly understood under natural conditions, particularly in wild raptors. Limited access to free-living nestlings constrains our understanding of how host species identity, parental trophic ecology (shaping dietary microbial input and nutrient availability), and environmental factors structure early-life microbiota. Here, we investigated the gut microbiota of nestlings of two sympatric raptor species, the white-tailed eagle (Haliaeetus albicilla) and the lesser spotted eagle (Clanga pomarina), which differ in parental foraging ecology (fish/waterbird vs. small mammal-based diet). Using non-invasive fecal sampling during routine ringing, we characterized gut microbiota composition and predicted functional potential, and tested the effects of host species, geographic distance between nests, and within-nest variability. Gut microbiota composition differed markedly between species, with clear separation of microbial communities and higher alpha diversity in white-tailed eagle nestlings. In contrast, geographic distance between nests had a limited influence on microbiome structure, while pronounced inter-individual and within-nest variability was observed, highlighting the importance of individual-specific and potentially stochastic processes during early microbiome assembly. Predicted functional profiles also differed between species, with enrichment of amino acid biosynthesis pathways in white-tailed eagles and carbohydrate-related pathways in lesser spotted eagles. No dysbiosis-like microbiome profiles were detected in either species, providing a baseline for future comparative studies. Overall, our findings demonstrate strong species-level differentiation in early-life gut microbiota in wild raptors, in the context of contrasting trophic ecology, while local environmental variation appears to be of secondary importance.