Published in HBW Volume 13 on page 220.
Original HBW caption:

As nectar-feeders, sunbirds play an important part in plant reproductive processes. In South Africa, for example, some 2% of all flowering plants are pollinated by sunbirds, which is a particularly impressive feat given the country’s remarkable plant diversity. As sunbirds insert the bill into the corolla to reach the nectar within, the forehead, lores or bill base touches the anthers and becomes dusted with pollen. When the bird moves to another flowering plant of the same species, it deposits the pollen grains, which contain the male gametes, on the recipient plant’s stigma, which leads to the ovules that house the female gametes in the ovary. As this male Whitebreasted Sunbird demonstrates, the buildup of pollen can be quite substantial, powdering a large area of the head.