The use of plastic debris as nesting material by a colonial seabird and associated entanglement mortality

► Plastic entanglement causes mortality but the population-level consequences are unknown. ► At one colony, northern gannets used 18.46tonnes of plastic for nesting material. ► Synthetic rope appears to be preferentially selected. ► On average 65.63 birds are entangled annually. ► Current levels of mortality are unlikely to have population-level effects. Entanglement with plastic debris is a major cause of mortality in marine taxa, but the population-level consequences are unknown. Some seabirds collect marine debris for nesting material, which may lead to entanglement. Here we investigate the use of plastics as nesting material by northern gannets Morus bassanus and assess the associated levels of mortality. On average gannet nests contained 469.91g (range 0–1293g) of plastic, equating to an estimated colony total of 18.46 tonnes (range 4.47–42.34tonnes). The majority of nesting material was synthetic rope, which appears to be used preferentially. On average 62.85±26.84 (range minima 33–109) birds were entangled each year, totalling 525 individuals over eight years, the majority of which were nestlings. Although mortality rates are high, they are unlikely to have population-level effects. The use of synthetic fibres as nesting material is a common strategy among seabirds, but the impacts of entanglement warrants further investigation.