Spatiotemporal differences in plastic biovectoring among three sympatric waterbirds

Abiotic vectors of plastic and their impact in natural areas have been extensively studied, whereas biotic vectors have received less attention. Recent studies demonstrate that birds can act as powerful agents of plastic transport, moving large quantities of plastic from landfills to natural habitats through a process called biovectoring, causing pollution hotspots. While most studies have focused on single species, the present research broadens this approach. Here we compared the quantity, composition and spatio-temporal variation of plastic biovectoring among three co-existing waterbird species foraging on landfills near a coastal wetland: white storks, yellow-legged gulls, and lesser black-backed gulls in Cádiz Bay Important Bird Area (CBIBA; 152 km²), Spain. We analysed 177 regurgitated pellets (42-74 per species), weighed their plastic content and used FTIR-technology to identify and classify polymer composition. We then characterized each plastic item by shape, size and colour, enabling interspecific comparisons using multiple correspondence analysis. Finally, we combined census, GPS data and the plastic obtained from the pellets to develop a daily plastic loading model for each species. We found that white storks transported the most plastic per pellet (0.14 g by median), compared to 0.034 g for yellow-legged and 0.026 g for lesser black-backed gulls. In general, gulls carried similar types of plastic items, with more film and larger sizes than storks. In total, 531 kg of plastics were estimated to be biovectored into the CBIBA from landfills in 2022, with deposition being higher in winter and lesser black-backed gulls transporting about 54 % of the total, followed by yellow-legged gulls with 30 % and white storks with 16 %. In addition, we also identified major seasonal and spatial differences among species. Our results highlight the importance of using a multi species approach to plastic biovectoring, which is essential to understand and estimate its environmental impact, and to inform management strategies.