Repeated measures of mercury concentrations demonstrate persistent territory-level contamination and stable individual differences in exposure in a riverine bird predator

Mercury is a persistent, toxic metal that bioaccumulates and biomagnifies in food webs, causing lethal and sublethal effects in wildlife even at low concentrations. It poses a pervasive threat to aquatic ecosystems through transformation into the highly toxic and bioavailable form, methylmercury. Yet the relative contributions of environmental and individual sources to variation in mercury exposure in wild populations remain unclear. We quantified total mercury concentration in feathers of adult female white-throated dippers ( Cinclus cinclus ) breeding along 11 rivers in the Chartreuse mountains, France, differing in anthropogenic pressures. Using 777 feathers from 451 adult females sampled up to three times from 2017 to 2025, we conducted repeatability analyses and linear mixed models to test the influence of landscape features on mercury exposure. Feather mercury averaged 1.62 ± 0.95 mg·kg⁻¹, with 0.8% of samples exceeding the 5 mg·kg⁻¹ toxicity threshold. When modelled separately, individual identity explained 80% of variance in mercury, territory identity explained 53%, and river identity explained 31%. When modelled together, variance was redistributed, with individual identity explaining 42% of variance in mercury, followed by river (26%) and territory (12%). Contrary to expectations, mercury was negatively associated with human footprint index. As white-throated dippers are highly faithful to their territory across years, our results suggest mercury exposure is shaped by persistent territory-level contamination and stable individual differences, with territory effects likely reflecting legacy contamination. These findings underscore the value of repeated sampling of known individuals and integrating human impact gradients to assess contaminant exposure risks in natural populations.