The seasonal threat of lead exposure in bald eagles.

Bald eagles often scavenge hunting remains embedded with lead bullet fragments, which debilitate and kill many eagles. Measuring blood lead concentrations (BLC) in free-flying bald eagles and those received by rehabilitators allows researchers to both actively and opportunistically monitor exposure. From 2012 to 2022, we captured 62 free-flying bald eagles and measured their BLC following the big-game hunting season in Montana, USA, which occurs from late October through late November. Between 2011 and 2022, we also measured the BLC of 165 bald eagles received by Montana's four raptor rehabilitation centers. Most of the free-flying bald eagles (89 %) had BLC above background (≥10 μg/dL), and BLC of juveniles tended to be lower as winter progressed (ρ = -0.482, P = 0.017). Bald eagles received by rehabilitators had an almost identical prevalence of BLC above background (90 %) over that same timeframe (n = 48). However, those eagles in rehabilitation were more likely to have BLC exceeding the clinical threshold (≥ 60 μg/dL), which we observed only from November through May. Between June and October, 45 % of bald eagles in rehabilitation had sub-clinical BLC (10-59 μg/dL), suggesting that many eagles may live with BLC chronically above background concentrations. Hunters may help lower BLC in bald eagles by switching to lead-free bullets. Those mitigation efforts could be evaluated through a continued monitoring of BLC in both free-flying bald eagles and those received by rehabilitators.