 Rivers that present a barrier for some species in a region may not necessarily do so for all, leading to species-by-species and clade, or genetically distinct group, differences in degree of isolation and differentiation on opposing shores.Large mammals and birds have little trouble crossing most streams, whereas small birds unaccustomed to long distance flight can have particular difficulties and thus may be more subject to population division.Additionally, rivers more effectively divide species that prefer terra firme forest as meanders and the process of oxbow formation in alluvial regions can narrow otherwise impassable streams.