Rapidly diversifying clades have traditionally been associated with narrow range-sizes and high specialization of species.  In birds, dispersal ability and breeding range size have been found to explain most of the variation in species richness.   We evaluated the relationship between species diversification and range sizes of ~1300 bird species, clustered in 175 bird sister genera endemic to the Americas and representing all major families of birds in the region.  We constructed a logistic model, testing the influence of the mean and variance in species range size and the interaction between the two for species diversification.  We asked: 1) Which combinations of mean and variance in range size accounted for the greatest species richness contrasts among sister groups?  2) What suit of species range size within clades is prevalent on species rich clades of neotropical birds?

Results/Conclusions

We found no correlation between species diversification and mean species range size, but diversification increased with the variance in species range sizes, and the interaction between mean and variance in range size was significant.  Small mean range size per species and no difference in range size variance was most strongly associated with diversification, but this pattern was observed in few sister taxa.    For more than half of the sister genera, greater variance in range size within clades was correlated with higher species richness.  Thus, clades with idiosyncratic range sizes, where closely related species have little similarity in range sizes, have diversified more than sister genera, who conversely have larger ranges and low variance in range sizes.  In conclusion, mean and variance in species range size within a clade can predict a large proportion of the difference in species richness between sister clades.