Aim: To investigate spatial variations in the ecological trait structure of breeding bird assemblages on oceanic islands. To test the hypothesis that native and naturalized alien bird species are filtered by different processes, leading to diverging associations between traits and insular environmental gradients.Location: Oceanic islands worldwide.Time Period: Current.Major Taxa Studied: Terrestrial breeding birds.Methods: We assessed the composition of breeding terrestrial bird assemblages from the extent-of- occurrence maps of 3170 native and 169 naturalized alien species on 4660 oceanic islands. We quantified their ecological trait structures with respect to diet, mobility and body mass as the standardized distance between a mean pairwise trait distance index and its expectation from a null model. We used spatial general-ized additive models to relate trait structures to proxies of environmental conditions and human impact on land, accounting for all species, native species only and alien species only.Results: Diet and mobility traits tended to be more diverse than expected by a null model, while body mass tended towards clustering. Trait-environment associations were idiosyncratic, but environmental variables tended to explain trait structures bet-ter than human impacts on habitats. Islands invaded by alien species had similar trait structures as noninvaded ones, although they hosted assemblages with more clus-tered body masses. However, trait-environment relationships diverged when consid-ering all islands and all species, invaded islands only or alien species only.Main Conclusions: Despite their ecosystem- level influences, alien species have a limited effect on the global patterns of trait structures in the breeding bird assem-blages of the world's islands, either because they account for a low proportion of species or because successful invaders and native species have similar trait suites. Trait-environment associations suggest that filters related to the conditions of alien species' introductions explain their distributions in island assemblages better than the constraints associated with isolated environments.