<p>Urbanization is a key driver of biodiversity change, yet little is known about the ecological patterns of overwintering bird communities and their drivers within boreal cities. Here, we assessed shifts in overwintering bird diversity in Lahti, southern Finland. Specifically, we analyzed city-wide changes in bird species richness and composition along an urbanization gradient, as well as the relationships between environmental and structural variables and recorded bird species richness and abundance. Our results show that overwintering bird species richness declines with increasing urbanization, with a pronounced drop beyond 60% built cover. Species abundance also decreased with urbanization, differing from previous findings in boreal regions. Most species recorded across our surveys were generalist granivores and omnivores, shared among survey sites. Although overall species composition showed moderately high dissimilarity, no clear patterns emerged across urbanization levels. Temperature and built cover were the only significant predictors of bird diversity. Temperature was positively associated with species richness, while built cover was negatively related with both species richness and abundance. Notably, our model for bird abundance had low explanatory power, suggesting unmeasured factors may influence patterns. Although our study is limited to a single city and year, it provides valuable insights into overwintering bird ecology in urban boreal environments. Our findings highlight the role of urban areas for overwintering birds and suggest that future research on mechanistic drivers, including human feeding, could inform urban planning strategies for non-urban bird populations.</p>