Dispersal distances predict subspecies richness in birds

Dispersal ability has been hypothesized to reduce intraspecific differentia tion by homogenizing populations. On the other hand, long-distance disperse rs may have better opportunities to colonize novel habitats, which could re sult in population divergence. Using direct estimates of natal and breeding dispersal distances, we investigated the relationship between dispersal di stances and: (i) population differentiation, assessed as subspecies richnes s; (ii) ecological plasticity, assessed as the number of habitats used for breeding; and (iii) wing size, assessed as wing length. The number of subsp ecies 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 o f ecological plasticity, suggesting that long-distance dispersers do not ha ve 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 population s and that low dispersal ability might promote speciation.