Microplastic exposure and body condition in White-throated dipper (Cinclus cinclus) nestlings across anthropogenic landscapes in Scotland and the Basque Country (Spain)

Microplastics (MPs) are widely distributed throughout freshwater ecosystems and are easily ingested by aquatic organisms. However, the uptake of MPs by freshwater birds and their potentially detrimental impacts on fitness and health remain poorly understood. We collected faecal samples from White-throated dipper (Cincluscinclus) nestlings across Scotland and the Basque Country (Spain), encompassing territories with varying degrees of anthropogenic land use. We analysed MP concentrations in the faeces alongside body condition, a proxy for growth conditions and health during early development. MPs, mostly fibres, were detected in 62.5% of broods, with concentrations positively correlated with urban and agricultural land cover. Despite this, MP load exhibited only a weak association with nestling body condition, likely reflecting underlying variation in diet quantity or quality. Overall, our findings highlight the widespread presence of MPs in freshwater birds inhabiting anthropogenic landscapes but indicate no short-term effects on nestling growth. Further research is needed to understand the long-term health implications of MPs and to determine optimal proxies for assessing their effects on freshwater bird health.