Gut microbiome variation in juvenile blue tits in a European urban mosaic.

Urbanisation transforms natural environments, impacting not only wild animals living in cities but also the microorganisms they are hosting. To better understand urban-driven variation in microbiological composition and diversity in the gut of birds developing in urban areas, we collected faecal samples from blue tit Cyanistes caeruleus nestlings using nestboxes distributed across the capital city of Warsaw, Poland. Sampling included a variety of urban habitats, a suburban village and a natural forest area. Microbiome analysis unveiled a pattern of reduced alpha and beta diversity in urbanised settings driven by impervious surfaces. Additionally, we observed that this effect was year-dependent, therefore highlighting the importance of temporal replication in ecological research. Furthermore, comparing two cavity types (natural and human-made), we demonstrated that artificial nestboxes, a tool widely used in field ecology, can impact the microbiome assembly in nestlings. Our results align with and expand upon earlier insights in great tit Parus major gut microbiome variation inferred in the same urban study system on a species ecologically related to the blue tit, confirming the pervasive impact of urbanisation on the avian gut microbiome.