Mercury and pesticides occurrence in white storks (Ciconia ciconia) feathers from areas with different land use.

White storks (Ciconia ciconia) are effective biomonitors of environmental pollution due to their wide distribution, opportunistic foraging behaviour, and tolerance of human-modified habitats. In this study, we measured Hg and 24 pesticides in the feathers of C. ciconia from multiple sites in north-western Spain with heterogeneous land use patterns. Our objectives were: (i) to quantify pollutant levels in feather samples; (ii) to assess internal variation in Hg accumulation between feather structures (rachis versus barbs); and (iii) to examine relationships between pollutant levels and surrounding land use. Hg was detected in all samples, with comparable concentrations in the rachis than in the barbs (up to ∼1400 ng g-1), indicating that both structures provide similar biomonitoring sensitivity. While most values fell within low to moderate risk thresholds, some colonies showed elevated levels of both Hg and pesticides. Legacy organochlorines, such as DDT, DDE, and dieldrin, were sporadically detected, highlighting their long-term persistence. Other pesticides, such as atrazine, gamma-hexachlorocyclohexane, and the synthetic repellent DEET (up to 18 ng g-1), were also found, reflecting ongoing or residual exposure. Higher concentrations of pollutants were associated with intensive agricultural areas, as revealed by PCA and Spearman's correlations. These spatial patterns indicate a possible influence of land use on pollutant exposure. Although the overall pollutant load was relatively low, their presence indicates a continuing environmental risk. Our findings support the usefulness of white storks as a sentinel species for terrestrial pollutants and demonstrate the potential of feathers for integrated monitoring. Combining biomonitoring with detailed land use data is a valuable tool for identifying exposure hotspots and informing targeted conservation and mitigation strategies.