Bird city life: tolerance to urbanization pressure shapes bird assemblage spatial patterns in a metropolitan landscape

ContextUrbanization acts as a selective filter for wildlife, favoring species pre-adapted to urban environments while creating hostile conditions for others. Previous studies have focused on which traits determine success in urban habitats, while little effort has been put into delineating how bird assemblages are spatially structured within the city.ObjectivesWe investigated how spatial gradients in anthropogenic and environmental variables shape bird assemblage distributions across Rome, one of Europe's largest and greenest metropolitan areas, and map their distribution to reveal how community structure shifts along the urbanization gradient.MethodsIn spring 2024 and 2025, we conducted acoustic monitoring in 102 sites across Rome's metropolitan area. Detections of 48 species were modeled against landscape variables, synthesized in three principal components reflecting urbanization intensity, forest cover, and isolation. We then used Outlying Mean Index analysis to cluster species with similar habitat use, and identified three distinct assemblages (avoiders, adapters, and exploiters), and mapped their distribution.ResultsOur results highlight a spatial turnover of bird compositions along the urbanization gradient, with different degrees of urbanization supporting distinct bird assemblages. avoiders were concentrated in extensive green areas at the city periphery, while adapters were largely distributed in transitional areas at the interface with the urban matrix. exploiters dominated the urban core but were mostly absent from green areas.ConclusionsVarying urban conditions shape distinct bird assemblages, composed of species with similar responses to urbanization and vegetation. Ensuring a permeable urban matrix and integrating greening initiatives within urban cores can contribute to maintaining high diversity, enhance ecosystem services, sustain species persistence, and limit disservices.