Individual and population effects of entanglement mortality on ospreys from plastic baling twine in nests

Abstract

The global production of plastics continues to increase, and most of our knowledge about the negative effects of plastics pollution comes from marine environments. The presence of plastics in nests of terrestrial birds has increased the last century, which can cause adult and nestling mortality through entanglement. Volunteers and I surveyed 600 km of the Yellowstone River, Montana, USA for breeding ospreys and we recorded the presence and amount of polypropylene baling twine in nests, and the number and outcome of nestling and adult entanglements from 2014 to 2022. Each year a mean 44.2 % of nests contained twine and a mean 3.4 % of nestlings became entangled. Entanglement mortality appeared compensatory rather than additive because the local breeding population increased each year and because annual reproductive success exceeded estimates to maintain a stable population. The amount of twine in 35 nests that were either destroyed by weather or removed by electric companies from energized power poles varied substantially (0–227 m). Entanglements occurred at 19 different nests, of which seven (36.8 %) were responsible for multiple entanglements (61.3% of all entanglements). Reducing osprey mortalities and baling twine litter in the environment will take short-term (intensive nest monitoring, bird rescues) and long-term actions (community education, cleanup, recycling).