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The density, biomass, species richness and composition of birds in plots in two Mountain Fynbos plant-species assemblages (Tall Mixed Fynbos and Restionaceous Tussock Marsh), infested with alien woody plants (mainly Australian Acacia spp.) at the Cape of Good Hope Nature Reserve, South Africa, were compared with those of uninfested plots. The relative density of frugivorous and granivorous birds tended to increase, but total bird density did not change significantly with increasing density of Acacia cyclops in Tall Mixed Fynbos. However, the density of nectarivorous birds decreased with increasing A. cyclops infestation as a result of the elimination of their food plants (proteaceous and ericaceous shrubs). Thus, alien woody plants have the capacity to disrupt the putative avian nectarivore – indigenous plant pollination relationship. Dense infestations of mixed alien species in Restionaceous Tussock Marsh supported fewer nectarivorous birds, but more granivores and insectivores than did uninfested vegetation. Moreover, bird species more typical of woodland and thicket (and absent from uninfested fynbos vegetation), notably large granivores, were found in dense mixed-alien infestations of this plant-species assemblage.