Characterization of plastic ingestion in urban gull chicks and its implications for their use as pollution sentinels in coastal cities

The increase of plastic pollution represents a significant ecological threat, particularly in human-impacted environments. However, the effects of plastic ingestion by urban wildlife are less understood. This study investigates the presence of microplastic (MPs; plastic <5 mm in size) and macroplastics (MaPs, plastic >5 mm in size) in yellow-legged gull (Larus michahellis) chicks inhabiting the urban marine ecosystem of Barcelona (northeastern Spain). The stomach contents of 56 gull chicks were analysed, revealing the presence of MPs in 100 % of the individuals and MaPs in 19.64 % of individuals. Additionally, trophic analysis, through stomach content and stable isotope determination, identified links between diet and plastic ingestion, with diet diversity associated with higher MaP abundance. These results highlight the high presence of plastics in the early stages of an urban-dwelling wildlife species and open the potential role of the use of urban gull chicks as sentinels of marine and terrestrial pollution in urban coastal areas. The findings suggest that chicks can serve as bioindicators of plastic pollution, emphasizing the urgent need to address the high levels of plastic contamination in urban environments.