Main conclusionsThe riverine dispersal boundary, floristic heterogeneity and forest fragmentation were associated with distinctive components of avian species compositional dissimilarity, collectively explaining three-quarters of the total dissimilarity among sites. Compositional dissimilarity was consistent with historical and continuing isolation of avian populations on opposite sides of the river, and may be partly driven by niche differentiation between subspecies. Geographical distance as a measure of dispersal limitation would not have accounted for these relationships. The use of rivers in biogeographical region delineation should address their variable importance for different taxa.