Evaluating the Ecotoxicological Effects of Microplastics on Terrestrial Passerines: Insights from Eurasian Tree Sparrows

Microplastic (MP) pollution poses a threat to wild animals, but its toxicological impact on terrestrial passerines remains unclear. To address this gap, we conducted the first systematic study investigating how microplastic particle size and dosage jointly influence gut microbiota and multi-system physiological functions in a small terrestrial bird. Eurasian tree sparrows (Passer montanus) were exposed to polystyrene microplastics (PS-MPs) of two particle sizes (0.5 and 15 μm in diameter) and two dosages (100 and 500 μg/d) via oral ingestion for 21 days. After exposure, body status, peripheral blood cell profiles, organ indices, intestinal histomorphology, oxidative stress, and barrier integrity markers displayed no significant changes compared with the control group. In the gut microbiota, large PS-MP particles significantly enhanced microbial species richness and phylogenetic diversity, and their effect was more pronounced than that of small ones. Additionally, structural alterations and distinct community compositions emerged across groups. Both particle size and dosage affected gut microbial composition and taxa abundance, with particle size exhibiting a relatively stronger effect. However, the relative abundance of the top 10 dominant phyla and predicted microbial functional profiles exhibited no significant intergroup differences. In summary, short-term PS-MP exposure primarily impacts the gut microbial structure of Eurasian tree sparrows without disrupting their key physiological functions. This suggests that the birds possess a certain buffering capacity against short-term PS-MP stress, though their long-term ecological tolerance to complex, real-world MP mixtures remains to be further investigated.