Urban metal pollution explains variation in reproductive outputs in great tits and blue tits

It is regularly reported that avian reproductive outputs are reduced in urban areas, yet the underlying reasons for discrepancies between urban and natural habitats are to date poorly explained. To address this knowledge gap, we tested whether the reproductive outputs of wild great tit (Parus major) and blue tit (Cyanistes caeruleus) populations in Warsaw (Poland) correlated with the concentrations of six main metallic/metalloid trace elements (MTEs; copper, zinc, lead, cadmium, arsenic and mercury) in three types of biological material pertaining to avian reproduction: nestling feathers, nest material and nestling droppings. For the first time, our study highlights consistent negative effects of copper and arsenic concentrations in nestling feathers on fledging success and nestling mass in both great tits and blue tits. Fledging success was also negatively correlated with cadmium and lead concentrations in nestling droppings. Importantly, while the relative proportions of each MTE were equivalent between the three biological materials, reproductive success correlated better with MTE concentrations in nestling feathers than in the two other materials; this result suggests that MTE absorption would explain part of the variation in individual fitness and emphasises the relevance of using nestling feathers for investigating the effects of MTE exposure on nestlings of hole-nesting birds. Altogether, our results suggest that urban MTE pollution likely contributes to the differences in reproductive outputs observed between tit populations living in urban and more natural environments.