Urbanization Affects Nest Material Composition and Use of Anthropogenic Materials in Blue Tit Nests (Cyanistes caeruleus)

Avian nests are complex structures that protect eggs and nestlings. Nest-building behaviour varies between species and habitats, and recent work has highlighted that in areas with high human activity and low availability of natural nest material, birds may use anthropogenic material to construct nests. However, we still know relatively little about how nest composition is affected by human presence, including along urban gradients. Here we present the results of a study where we examined how nest composition differed between an urban and a forest population of blue tits, and the impact that variation in nest composition had on reproductive success. We found a significant decrease in the amount of natural materials and an increase in anthropogenic materials in urban compared to forest nests. Earlier urban nests showed a higher amount of anthropogenic materials compared to late urban nests. The amount of moss incorporated in a nest was the strongest predictor of fledgling success, suggesting that the use of anthropogenic material by urban birds might be a maladaptation, and/or that urban birds are constrained in the amount of moss they can use to build their nests. Future studies should aim at quantifying the availability of natural materials to distinguish between these two hypotheses. ### Competing Interest Statement The authors have declared no competing interest.