Are plastic particles reduced in size in seabirds' stomachs?

Many studies have quantified plastic loads in seabirds' stomachs but provide imprecise estimates of retention time. Quantifying retention time is fundamental for assessing biological impact and for the use of plastic as bioindicators of plastic pollution. Presumably, plastic is retained in the grinding section of the stomach (gizzard or ventriculus) until ground small enough and particles then pass into intestines and are defecated. We assume that particle size is related to time in the ventriculus and compared size of plastic particles in juveniles, which were exposed to plastic for a known time, to non-juveniles which were exposed for an unknown longer period. Plastic particles from Northern Fulmars (Fulmarus glacialis) were of similar size between groups suggesting similar wear and retention time. Cassin's Auklet (Ptychoramphus aleuticus) juveniles contained larger particles than adults suggesting particles retained in adults were worn down and thus held for a longer period. •Many studies have quantified plastic loads in seabirds' stomachs.•Assessing impact of ingested plastic would benefit from retention time estimates.•Size of plastic particles is assumed to be related to wear and retention times.•We compared particle size relative to retention time in two seabird species.•Relative to time, particle volume was reduced in one species, but not in the second.