• Urban forest patches in Ushuaia are the southernmost in the world. • Urbanization enables openland birds in urban patches, uncommon in non-urban forests. • Urbanization favours abundance increment of omnivorous and granivorous bird species. • Proximity to non-urban forests and native plants help keep native birds in cities. Rapid urban development threatens biodiversity, reducing local livelihoods and well-being when replacing natural habitat by urban structures. Although there is a lot of literature on the effects of urbanization on biodiversity, little has been investigated in South America. This study evaluated, for the first-time, bird community variation in an urban gradient of Nothofagus South-Antarctic forest in Ushuaia, the southernmost city of Argentina. We also studied differences during the breeding season and habitat characteristics. We conducted bird surveys monthly from October 2016 to February 2017 in 12 plots classified as (i) urban, (ii) peri-urban and (iii) non-urban forest. We counted birds for 15 min (three times per month) in each plot to obtain assemblage richness, abundance, density and several indices (Shannon-Weiner diversity, Pielou evenness and abundance-based Chao-Sorensen similarity) for the communities and functional groups. We recorded use of strata, weather conditions and human/domestic animal presence during counting, as well as habitat variables (forest structure, ground cover and understory plants) in each plot. We performed univariate and multivariate analyses, which showed that urbanization positively affects bird community structure (all variables increased in urban forest), with a reduction in the abundance of insectivores cavity-nesters that feed on trees (Campephilus magellanicus , Troglodytes aedon, Aphrastura spinicauda and Pygarrhichas albogularis). Bird assemblages changed similarities over the season; non-urban forests were more dissimilar than urban and peri-urban in October and November, but not in the other months, highlighting different roles of each forest in different periods. We found differences in habitats along the urbanization gradient, and discuss possible influences on bird communities, such as more food supply associated with berry shrubs and low overall richness at high latitudes. According to our results, urban forests with native vegetation are useful structures that help to maintain native bird communities and ecological functions in Ushuaia city. [ABSTRACT FROM AUTHOR]