A Review of Microplastic Presence in Birds: Research Advances and Socioeconomic Contexts

Plastics have become deeply embedded in modern society, with global production increasing more than 230-fold over the past seven decades, raising major ecological concerns. Plastic disperses across terrestrial, freshwater, and marine ecosystems via human activity and natural forces, fragmenting into microplastics ingested by birds, key bioindicators. This study synthesizes, for the first time, peer-reviewed research on microplastic contamination in 129 bird species to evaluate particle characteristics, prevalence, and global research patterns. Results show a strong bias toward aquatic birds, with far fewer studies addressing terrestrial taxa, although a wide range of polymers, colors and morphotypes has been reported across avian tissues, pellets, feces, regurgitates. When correlated with national indicators, research output was highly uneven geographically and concentrated mainly in large or highly populated countries such as the United States, China, India, Brazil, Canada, and Australia. Productivity was more strongly associated with scientific capacity indicators—GDP, country surface area, and the number of top-ranked universities—than with population size or pollution intensity. Generalized Linear Model analysis revealed that only per capita plastic waste showed a significant, negative association with publication number, indicating that high environmental contamination does not necessarily promote monitoring efforts. Overall, the findings reveal a structural imbalance: countries generating the most plastic waste often conduct the least microplastic research on birds, likely reflecting research and funding differences rather than environmental relevance. Expanding monitoring frameworks, particularly in regions with high pollution but low research capacity, is crucial to advance global understanding of avian microplastic exposure and inform mitigation strategies.