Non-native species are one of the greatest threats to biodiversity worldwide, due to their direct and indirect effects on native communities. There are two opposing hypotheses to explain how non-native species successfully establish outside their native range. The first posits that non-native species are closely related to local native species through environmental filtering which selects species with similar traits; the second, that non-native species are distantly related to native species in the area in which they establish due to limiting similarity which minimizes competition. We assessed support for these two hypotheses by characterising the functional trait space of bird communities in Italian cities. We surveyed 220 points in breeding and winter periods along an urbanisation gradient in six cities. We assessed the two opposing hypotheses by calculating functional diversity metrics (Functional Dispersion i.e. quantification of the distribution of functional elements in the niche space, and Contribution i.e. contribution of each species to the niche space) for each community. We then modelled these metrics in relation to the presence/absence of non-native parakeet species along the urbanisation gradient. We found that non-native parakeet species were more likely to establish in the vacant functional niche space of urban bird communities, showing marked dissimilarity to native species in terms of niche space. Our results suggest that limiting similarity is the main mechanism promoting invasions at the local scale. Urban environments offer novel opportunities that are exploited by non-native birds, minimising competition with native species. This insight into niche space processes in urban areas, which can act as centres for expansion of non-native birds into other environments, can be used when implementing management strategies to enhance environmental filtering in these areas, thus reducing the chances of further establishment of non-native species.