Abstract:
Secondary compounds in nectar may play a decisive role in determining the
spectrum of floral visitors on plants. Flowers of the African coral tree Erythrina caffra are
visited mainly by generalist passerine nectarivores, such as weavers and bulbuls. As the
nectar of this species tastes very bitter to humans, it was hypothesized that secondary
compounds may repel sunbirds and honeybees which are common in the same habitats yet
seldom consume the nectar. We conducted choice tests using fresh nectar and both sucrose
and hexose (glucose/fructose) solutions of the same concentration as the nectar. Whitebellied
Sunbirds (Cinnyris talatala) were repelled by nectar of both E. caffra and a related
species Erythrina lysistemon, but Dark-capped Bulbuls (Pycnonotus tricolor) did not
discriminate between the Erythrina nectar and control sugar solution in terms of amounts
consumed. Honeybees (Apis mellifera scutellata) probed exposed droplets of E. caffra
nectar and a control sugar solution at the same rate, suggesting that there is no volatile
deterrent, but they immediately withdrew their proboscis far more often from the droplets
of Erythrina nectar than they did from the sugar solution, suggesting that they find Erythrina
nectar distasteful. These results contribute to a growing awareness that non-sugar
components of nectar can play important functional roles in plant pollination systems.
