Longitudinal trends of metal(loid)s in nestling bald eagles (Haliaeetus leucocephalus) from Wisconsin

Bald eagles (Haliaeetus leucocephalus) are valuable bioindicators of ecosystem health due to their high trophic position, well-monitored populations, and reliance on aquatic food webs. Therefore, we assessed concentrations of three metals and one metalloid ("metal(loid)s")-lead (Pb), mercury (Hg), cadmium (Cd), and selenium (Se), respectively-in whole blood and feathers of nestling bald eagles collected from eight regions in Wisconsin between 2011 and 2024. A total of 151 nestlings from 106 territories were sampled. Concentrations of metal(loid)s in blood exhibited significant spatial variation, with Pb highest in the Fox River/Lower Green Bay, Hg highest in the Upper Wisconsin River, Cd highest in the Northern Highlands, and Se was highest in the Apostle Islands. Concentrations of metal(loid)s in feathers showed fewer spatial differences but correlated significantly with blood concentrations for Pb, Hg, and Se. Pb in blood significantly declined with nestling age and mass, while Se in feathers decreased with age. Temporal analyses revealed region-specific trends, including decreasing Pb and increasing Hg in the Northern Highlands, minimal increasing of Cd in Upper Green Bay/Lake Michigan, and increasing Se in the Fox River/Lower Green Bay. Concentrations of Pb and Hg in blood were generally below established toxicity thresholds, though Hg hotspots were observed in the Upper Wisconsin River region. Our findings confirm that bald eagle nestlings can act as sensitive sentinels for monitoring these legacy contaminants. Long-term biomonitoring across spatial gradients is essential for identifying contaminant hotspots, informing wildlife management, and safeguarding ecosystem health in the Great Lakes region.