Diet composition and related metal exposure in insectivorous passerines inhabiting metal-polluted areas

Anthropogenic pollution impacts insectivorous birds through both direct chemical exposure and indirect trophic effects. Environmental contamination alters plant and invertebrate communities, causing dietary shifts in birds inhabiting polluted environments. We used nestling feces to examine metal exposure, prey composition and fledging success of <i>Parus major</i>, <i>Cyanistes caeruleus </i>and<i> Ficedula hypoleuca</i>, across control and metal-polluted zones near a copper-nickel smelter in Finland. Taxonomic identification with DNA metabarcoding revealed four arthropod classes, 14 orders, 65 families, 157 genera, and 162 species of arthropods across all the birds’ diets. Orders Hymenoptera, Lepidoptera, Diptera, Coleoptera, Hemiptera and family Formicidae dominated both zones. <i>P. major</i> diet exhibited the lowest Shannon diversity due to Lepidoptera specialization, while <i>F. hypoleuca</i> showed the highest observed richness, reflecting opportunistic foraging. Species-specific dietary profiles revealed pronounced shifts from control to polluted zones: <i>P. major</i> reduced Lepidoptera while increasing Diptera and Coleoptera; <i>F. hypoleuca</i> shifted balance from Hymenoptera to Lepidoptera and Diptera; <i>C. caeruleus</i> maintained a diverse diet with no single order dominating either zone. Permutational multivariate analysis of variance confirmed distinct diets among species in the control zone, but pollution significantly reduced interspecific dietary differences. Pianka's Index revealed substantial dietary overlap increases under pollution, most notably a fivefold increase between <i>C. caeruleus</i> and <i>F. hypoleuca</i>, demonstrating dietary convergence among species. Fledging success of <i>P. major</i> was negatively associated with metal levels and caterpillar scarcity; <i>C. caeruleus</i> maintained dietary evenness across zones, and fledging success was positively associated with dietary evenness; <i>F. hypoleuca </i>was resilient to variation in metal levels but showed reduced fledging success in the polluted area despite increased dietary richness, suggesting reliance on less optimal prey. Together, these results demonstrate that heavy metal pollution restructures food webs, forcing higher dietary overlap and potentially escalating interspecific competition for remaining high-quality resources.