Gut microbiota divergence and pathogen transmission risk between invasive monk parakeets and native sympatric hosts

Background Pathogen transmission is a major mechanism by which invasive alien species (IAS) threaten native wildlife. The invasive monk parakeet ( Myiopsitta monachus ) reaches high urban population densities and builds communal nests that facilitate cohabitation with native birds. Despite their recognised role as potential carriers of diverse pathogens, empirical studies evaluating horizontal transmission between invasive monk parakeets and native avifauna remain scarce. Here, we aimed to investigate whether these spatial and physical overlaps promote horizontal pathogen spillover by characterising the overall gut microbial and potentially pathogenic communities of monk parakeets and two native species: stock doves ( Columba oenas ) co-nesting in parakeet structures, and Eurasian tree sparrows ( Passer montanus ) foraging in invaded and non-invaded sites. To achieve this, we sampled 117 nestlings in Madrid (Spain), combined 16 S rRNA gene amplicon sequencing with targeted qPCR viral screenings, and implemented iterative PERMANOVA alongside generalised linear mixed models (GLMMs) and differential abundance analyses. Results Regarding bacterial assemblages, invasive parakeets harboured a significantly simplified, Lactobacillus -dominated core bacteriome compared to native hosts, indicative of a microbial founder effect and strict physiological filtering. Multivariate analyses demonstrated that overall and potentially pathogenic communities were strictly segregated by host identity. Crucially, regardless of whether native birds shared communal nests (stock doves) or foraging grounds (tree sparrows) with the invader, they exhibited no increased pathogen overlap, altered bacterial loads, or structural microbiome changes associated with parakeet presence. Instead, univariate models confirmed strong host-specific filtering: several potentially pathogenic taxa exhibited complete mathematical separation driven by strict host-specific exclusions (e.g., Mycobacterium lineages being exclusive to sparrows), while other key lineages ( Pantoea , Stenotrophomonas , Rickettsiella , and Nocardia ) were significantly enriched in native tree sparrows relative to the invasive parakeet. Finally, all four targeted high-risk viral pathogens tested negative across all samples. Conclusions Strong phylogenetic and physiological filters determine the “parakeet microbiome,” preventing widespread bacterial spillover to the native avifauna despite intimate co-nesting and spatial sympatry. Consequently, our results do not support the view of these invaders as major disease transmitters in urban ecosystems. This empirical evidence provides critical insights for managers to avoid overestimating disease risks, allowing them to accurately adapt control measures to the actual threats posed by the species.