Dispersal ability has been hypothesized to reduce intraspecific differentiation by homogenizing populations. On the other hand, long-distance dispersers may have better opportunities to colonize novel habitats, which could result in population divergence. Using direct estimates of natal and breeding dispersal distances, we investigated the relationship between dispersal distances and: (i) population differentiation, assessed as subspecies richness; (ii) ecological plasticity, assessed as the number of habitats used for breeding; and (iii) wing size, assessed as wing length. The number of subspecies was negatively correlated with dispersal distances. This was the case also after correcting for potential confounding factors such as migration and similarity due to common ancestry. Dispersal was not a good predictor of ecological plasticity, suggesting that long-distance dispersers do not have more opportunities to colonize novel habitats. Residual wing length was related to natal dispersal, but only for sedentary species. Overall, these results suggest that dispersal can have a homogenizing effect on populations and that low dispersal ability might promote speciation.