Anthropogenic diets and spatial behaviour predict plastic ingestion in urban herring gulls: Insights from pellet...

Coastal environments are rapidly transforming under increasing anthropogenic influence, reshaping patterns of pollutant exposure in wildlife, particularly for opportunistic seabirds such as gulls that forage across urban, agricultural, and natural coastlines. Individual differences in foraging and movement strategies then structure exposure pathways within colonies, as birds frequenting human-modified habitats or exhibiting lower site fidelity potentially encounter a broader array of anthropogenic debris than habitat specialists that primarily exploit natural prey. Urban-adapted herring gulls (Larus smithsonianus) in the St. Lawrence Estuary can encounter a mosaic of human-derived and natural food sources, creating heterogeneous exposure pathways within a single colony. We combined pellet analysis of >350 regurgitated pellets with high-resolution GPS tracking over two breeding seasons to understand how adult foraging behaviour and space-use were associated with anthropogenic debris in pellets collected at breeding nests. Overall, 40% of pellets contained plastics, dominated by polystyrene and polyethylene terephthalate in anthropogenic diets, whereas agricultural pellets contained primarily polyethylene, reflecting distinct exposure pathways. Debris occurrence, count, and mass were highest in pellets reflecting anthropogenic foraging, while aquatic and agricultural diets consistently contained lower debris loads. GPS-derived metrics indicated that foraging strategy, not landfill use, predicted pellet-derived debris loads at the nest-level; nests associated with adults exhibiting lower site fidelity produced pellets containing more total debris and plastics. Non-plastic debris occurred in 21% of all pellets (80/376), and showed weaker associations with spatial metrics than plastic, suggesting primarily incidental ingestion. We conclude that herring gulls are sentinels of human-derived debris exposure, with plastic ingestion shaped by anthropogenic foraging and individual spatial behaviour, providing insights relevant to urban and municipal waste management.