Tire-Road-Wear Particles and Glass Beads in the Gizzard of the Endangered Terrestrial Bird, Okinawa Rail (Hypotaenidia okinawae)

This study analyzed microplastics and glass beads (GBs; an indicator of traffic-derived particulate contamination) in 42 gizzards of an endangered terrestrial bird, the Okinawa rail (Hypotaenidia okinawae). Black rubber fragments (BRs) were found in 57% of the specimens (1–184 items/individuals), and GBs were found in 48% (1–32 items/ind.). FTIR identified the rubbers as the same materials used in tire tread. A significant positive correlation was found between abundances of BRs and GBs in the gizzards (p < 0.01), suggesting the same source and exposure pathway. Large quantities of BRs, namely tire-road-wear particles (TRWPs), were also found in environmental samples (road dust, roadside soil, and side-ditch sediment) and diet organisms of the Okinawa rail (earthworm, millipede, and snail). The characteristics of these particles (appearance, material type, and size distribution) were consistent between the gizzard contents and the environmental samples. The concentration ratio of GBs and BRs in the Okinawa rail was similar to that in side-ditch sediment and diet organisms, especially earthworms. These results indicate that Okinawa rails are exposed to traffic-derived contaminants including TRWPs via ground-feeding along the roadside. To our knowledge, this is the first evidence of TRWPs exposure in terrestrial bird species.