The extended avian urban phenotype: anthropogenic solid waste pollution, nest design, and fitness.

Solid waste pollution (garbage discarded by humans, such as plastic, metal, paper) has received increased attention given its importance as a global threat to biodiversity. Recent studies highlight how animals incorporate anthropogenic materials into their life-cycle, for example in avian nest construction. While increasingly monitored in natural areas, the influence of solid waste pollution on wildlife has been seldom explored in the urban habitat. There is limited data on the relationship between anthropogenic solid waste pollution, nest design, and reproductive success in an urban context. We address this knowledge gap (i) by investigating the presence of environmental solid waste pollution in the breeding habitats of great tits Parus major and blue tits Cyanistes caeruleus reproducing in a gradient of urbanisation, and (ii) by quantifying (ii) the contribution of different anthropogenic materials in their nests. We further examine potential drivers of solid waste pollution by inferring three distinct properties of the urban space: environmental solid waste pollution on the ground, human presence, and the intensity of urbanisation (e.g impervious surfaces) in nestbox vicinity. Finally, (iii) we explore the relationship between anthropogenic nest materials and reproductive success. We found that environmental solid waste pollution was positively associated with human presence and urbanisation intensity. There was also a positive relationship between increased human presence and the amount of anthropogenic materials in great tit nests. Interestingly, in both species, anthropogenic nest materials covaried negatively with nest materials of animal origin (fur and feathers). We suggest that fur and feathers - key insulating materials in nest design - may be scarcer in areas with high levels of human presence, and are consequently replaced with anthropogenic nest materials. Finally, we report a negative relationship between anthropogenic nest materials and blue tit reproductive success, suggesting species-specific vulnerability of urban birds to solid waste pollution.