Urban green infrastructure fails to mitigate exposure to mercury with increasing pollution levels: evidence from corvids

The urban landscape acts as a novel habitat providing opportunities for certain species. However, populations of these so-called urban winners are constantly exposed to environmental contaminants, among which mercury (Hg) is perceived as a significant health concern for both wildlife and humans. While absorption of Hg is primarily related to its environmental level, it could be theoretically mitigated by near-natural habitats that have persisted within the urban landscape. Here, we investigated this hypothesis using three sympatric corvids as a model – Magpie Pica pica , Jackdaw Corvus monedula and Rook Corvus frugilegus . Total Hg concentrations were identified in the feathers (in the shafts and barbs separately) of each species collected in their breeding territories located along the urbanization gradient of Kraków (Poland). These concentrations were then correlated with particulate matter emissions (PM ₁₀ ) and habitat features (green and grey infrastructure) measured in the territories where the feathers had been collected. We expected that Hg concentrations in feathers would increase with increasing local pollutant emissions but decrease with increasing areas of green infrastructure, i.e. natural or semi-natural vegetation. Mean Hg concentrations in both feather shafts and barbs differed between the species. Magpie showed the highest concentrations (0.621 ±0.442 SD µg/g in barbs), followed by Jackdaw (0.441 ±0.272 µg/g) and Rook (0.296 ±0.134 µg/g). A generalized linear model yielded a correlation between habitat composition and Hg concentration only for Jackdaw feather shafts. A generalized additive model, however, revealed a non-linear relationship between PM ₁₀ emissions and Hg concentrations in feather barbs and/or shafts of Jackdaw and Magpie (but not Rook). Hg concentrations initially increased, but then fell with increasing PM ₁₀ emissions; these relationships were not influenced by habitat features. In conclusion, we found no evidence that Hg contamination was mitigated by urban green infrastructure.