Incorporation of plastic debris into the nests of great cormorants (Phalacrocorax carbo sinensis) in the Gulf of Finland

Several bird species are known to utilise marine debris among their nest materials in different parts of the world. This study examines the prevalence of debris in cormorant nests in the Gulf of Finland, focusing mainly on plastic debris. 200 nests were sampled on four different nesting islets, and plastic debris in the nest was counted and classified according to their type, colour, origin and polymer type. In total, 58 % of the nests contained debris (95 % plastics), but the prevalence of debris in the nests varied between the colonies. Core nests contained more debris than periphery nests. The relative number of threadlike plastics in the nests was higher than that in the surrounding environment, indicating active selection by cormorants. Based on the results, nest surveys could be a useful tool in evaluating the effectiveness of certain reduction measures aiming to tackle marine plastic pollution in the Baltic Sea. •58 % of 200 studied cormorant nests contained debris.•Plastics constituted the vast majority (95 %) of the nest debris.•Core nests contained significantly more debris than periphery nests.•Plastic debris originated from both marine and land-based sources.•Cormorants actively selected for threadlike plastics as nesting material.