A large diversity of floral morphological types exists within the bird pollination syndrome in the Cape Floral Region of South Africa. Here we focus on species with tubular flowers and ask: “Do differences in tube length among plant species correspond to differences in bill length among the birds that serve as pollinators?” To answer this question, we observed 1669 bird–plant interactions at thirteen bird-pollinated plant species across a spectrum of tube lengths. Only three nectar-feeding bird species pollinated the focal plant species. The relatively short-billed species were the Southern Double-collared Sunbird (Cinnyris chalybea) and Orange-breasted Sunbird (Anthobaphes violacea); the only long-billed species was the Malachite Sunbird (Nectarinia famosa). We found that the groups of plant species pollinated by the long- and short-billed birds differed significantly in floral tube length. Short-billed sunbirds nevertheless often robbed long-tubed flowers by piercing the corolla, and there was a significant difference in floral tube length among those species that were robbed, and those that were pollinated by short-billed sunbirds. The presence of territorial Malachite Sunbirds depressed robbing rates by short-billed sunbirds at long-tubed flowers. In turn, the absence of Malachite Sunbirds from short-tubed plant species might be explained by the observed positive correlation between tube length and nectar volume. Together, these data suggest that there is a subset of the bird-pollinated plants at the Cape that are pollinated solely by the long-billed Malachite Sunbird, a pollination service irreplaceable by the more abundant, short-billed sunbird species. To extrapolate this finding to a greater subset of species in the Cape Floral Region, we measured flowers on herbarium specimens of all tubular putatively bird-pollinated plants. We find that floral tube length has a bimodal distribution with 37 Cape species potentially dependent on pollination by Malachite Sunbirds.