Mercury in Red Knots (Calidris canutus rufa): Inter-Annual and Geographic Variation in Blood Mercury and Associated Toxicological Risk Along the Atlantic Flyway.

Shorebird populations are declining sharply, and trace metal contaminants may compound the many migration-related threats they face by affecting feeding, migration, and reproductive success. Despite this risk to the federally threatened Red Knot (Calidris canutus rufa), blood mercury in Red Knots has not been examined outside Delaware Bay, and the associated toxicological risk has not been assessed across the annual cycle. We report blood mercury from four cohorts: spring 2024 and spring 2025 at Delaware Bay, fall 2024 at Avalon, New Jersey, and spring 2025 at Kiawah Island, South Carolina. Blood mercury was substantially higher in 2024 than in 2025 across both cohorts. In 2024, 38% of spring Delaware Bay birds and 62% of fall Avalon birds exceeded the 200 ng/g adverse sublethal risk threshold; no birds sampled in 2025 exceeded this level. No biological endpoints were measured, so this risk is inferred from published thresholds. Within single-day Delaware Bay flocks, body mass and mercury were positively related, an association consistent with bioaccumulation. Two cohorts had small samples, and site, season, and migratory stage are confounded in the design. Risk varies markedly among years, and a single sampling event is unlikely to characterize mercury exposure or its associated risk.