Birds of a feather eat plastic together: high levels of plastic ingestion in Great Shearwater adults and juveniles across their annual migratory cycle.

Limited work to date has examined plastic ingestion in highly migratory seabirds like Great Shearwaters (Ardenna gravis) across the their entire migratory range, although this species is prone to ingest plastic as a wide-ranging procellariiform. We examined 217 Great Shearwaters obtained from 2008-2019 at multiple locations spanning their yearly migration cycle across the Northwest and South Atlantic to assess accumulation of ingested plastic as well as trends over time and between locations. A total of 2,328 plastic fragments were documented in the ventriculus portion of the gastrointestinal tract, with an average of 9 plastic fragments per bird. The mass, count, and frequency of plastic occurrence (FO) varied by location, with higher plastic burdens but lower FO in South Atlantic individuals from the breeding colonies. No fragments of the same size or morphology were found in the primary forage fish prey, the sand lance, (Ammodytes spp., n = 202) that supports Great Shearwaters in Massachusetts Bay, USA, suggesting the birds directly ingest the bulk of their plastic loads rather than accumulating via trophic transfer. Fourier-transform infrared spectroscopy indicated that low- and high-density polyethylene were the most common polymers ingested, within all years and locations. Individuals from the South Atlantic contained a higher proportion of larger plastic items and fragments compared to juveniles and non-breeding adults from the NW Atlantic, possibly due to increased use of remote, pelagic areas subject to reduced inputs of smaller, more diverse, and potentially less buoyant plastics found adjacent to coastal margins. Different signatures of polymer type, size, and category between similar life stages at different locations suggests rapid turnover of ingested plastics commensurate with migratory stage and location, though more empirical evidence is needed to ground-truth this hypothesis. This work is the first to comprehensively measure the accumulation of ingested plastics by Great Shearwaters over the last decade and across multiple locations spanning their yearly trans-equatorial migration cycle, and underscores their utility as sentinels of plastic pollution in Atlantic ecosystems.