Mercury exposure and isotopic signatures in overwintering Black-headed Gulls (Chroicocephalus ridibundus): Age-related variation and origin tracing.

Black-headed Gulls (Chroicocephalus ridibundus), a long-distance migratory bird species, serve as unique spatial integrators of mercury (Hg) pollution across their various geographical range. This study investigated the Hg exposure, Hg isotopic signatures, and trophic dynamics in the feathers of wintering Black-headed Gulls (70 adults, 28 juveniles) at Dianchi Lake in Southwest China. Results showed that adults had significantly higher concentrations of total Hg (THg: 4.4 ± 2.4 mg/kg) and methylmercury (MeHg: 3.3 ± 1.9 mg/kg) than juveniles (THg: 3.3 ± 1.6 mg/kg; MeHg: 2.3 ± 1.2 mg/kg). Approximately 35.7 % of adults (25/70) and 10.7 % of juveniles (3/28) exceeded the toxicological threshold (5.0 mg/kg). Stable isotope analysis (δ15N, δ13C) indicated that adults forage at higher trophic levels and have more diverse dietary sources than juveniles. Mercury isotopic compositions (δ202Hg: -0.35 ± 0.87 ‰ in adults, -0.48 ± 1.40 ‰ in juveniles; Δ199Hg: 1.18 ± 0.59 ‰ in adults, 1.14 ± 0.62 ‰ in juveniles) revealed that feathers Hg isotope signatures are consistent with integration from both inland breeding grounds and major stopover sites, reflecting the integration of Hg from geographically distinct sources along their migration routes. In conclusion, the Black-headed Gulls spatially integrate Hg signals from distinct ecosystems along their migration routes, provide a broad-scale perspective for tracking Hg sources and biogeochemical cycles.