Mining contamination is a widespread ecological disturbance with disproportionate effects on river and riparian ecosystems which act as strongholds of breeding songbird abundance and diversity. Riparian songbirds rely heavily on insects to fuel energetically expensive breeding activities, and emergent aquatic insects are superior fuel due to their high polyunsaturated fatty acid content. Elevated heavy metals and metalloids (metals) from mine waste can cause insect mortality during metamorphosis, limiting prey availability, especially for high-quality aquatic prey. Simultaneously, surviving adult aquatic insects, terrestrial insects, and spiders pose a risk of toxic metal exposure. Thus, in mine-waste contaminated habitats, diet mediates a dual risk of metal exposure and nutritional stress, but the relative importance of these stressors for riparian songbirds remains poorly resolved.
Nestling songbirds act as integrators of their environment; rapid growth is fueled by local insect prey, and condition and physiology are sensitive to developmental stressors. To explore the diet-mediated effects of mine-waste contamination, we assessed nestling body condition (fat score and size-corrected mass), telomere length (biomarker of health correlated with lifespan; qPCR), diet composition (DNA metabarcoding), and blood metal concentration (Pb, As, Cd, Cu, Zn, Se; ICP-MS) in four riparian songbird species across the heavily contaminated Upper Clark Fork River watershed in 2022–2023.
Blood concentrations of Pb, As, Cd, and Se were highest at the most contaminated sites, with species-specific differences in blood metals consistent with predicted reliance on aquatic versus terrestrial prey. Hierarchical Bayesian structural models revealed nuanced, site-specific relationships between diet, metals and condition variables. In some sites, higher reliance on aquatic prey was associated with higher fat scores and lower metal accumulation but surprisingly, we saw no associations between condition variables and the first principal component (PC) of metal variation (representing higher accumulation of all six metals). Instead, condition was negatively associated with PC2, where higher lead and copper predicted poorer condition, and higher selenium and arsenic predicted better condition, suggesting a potential protective role of selenium via antioxidant defense. Together, these results highlight the complexity of metal mixtures as stressors for riparian songbirds and the importance of diet-mediated effects in mining-contaminated systems.