Interspecific differences in microplastic accumulation and polymer profiles from regurgitated pellets of coexisting raptors

Microplastics have emerged as pervasive environmental contaminants, raising concerns over their impacts on wildlife. Birds of prey occupy high trophic positions and hence are particularly vulnerable to accumulating high contaminant levels due to potential biomagnification. We used regurgitated pellets as a non-invasive proxy for dietary microplastic exposure in two apex predators that share overlapping habitats in the United Kingdom (UK): barn owls (Tyto alba) and common kestrels (Falco tinnunculus). We collected regurgitated pellets (n = 24 per species), which were chemically digested and microplastics were enumerated and identified using Fourier-transform infrared spectroscopy (FTIR), and sorted into five size classes ranging from 25 to 200+ μm. Overall, 92% of kestrel pellets and 83% of barn owl pellets contained microplastics. Kestrel pellets had 18.46 microplastic particles g^-1, which was significantly higher than in barn owl pellets, which had 0.42 particles g^-1 (p = 0.03). Of a possible 20 polymers, 13 were detected with significant variations in occurrence between the two species. Overall, the dominant polymers were polypropylene (8.2%), polyurethane (9.2%), polyethylene (36.8%), ethylene vinyl alcohol (3.1%) and ethylene vinyl acetate (37.3%). Additionally, there were significant differences in number of microplastic particles between repeat sampling of the same nests. The largest number of particles were found in the 25-50 μm range, with 25 μm being the detection limit of our FTIR instrument. These findings demonstrate species-specific microplastic exposure, which may be driven by the variable microplastic load of different prey. To our knowledge, this study is the first to investigate microplastic ingestion by birds of prey in the UK.