Pink flower preference in sunbirds does not translate into plant fitness differences in a polymorphic Erica species

Bird-pollinated plants typically have reddish flowers, but it is not clear whether this trait can be attributed to selection by birds. Here we experimentally test for the first time the foraging behaviour of sunbirds in relation to flower colour, using the Orange-breasted Sunbird Anthobaphes violacea (Nectariniidae) and the colour dimorphic Erica perspicua (Ericaceae). Pink and white flower morphotypes co-flower in intermixed populations and have similar nectar volumes and concentrations. Using floral arrays in a field aviary, we found that sunbirds preferred pink flowers; 95 % of their first choices were to pink inflorescences and they visited and probed more pink inflorescences and flowers, respectively. We also tested for flower constancy (the tendency to move between same colour rather than different colour morphotypes), but found no evidence for this in the sequence of their foraging choices, indicating that this mechanism did not maintain flower colour differences in sympatry. There was evidence for optimal foraging: 80 % of moves were to adjacent inflorescences. Unexpectedly, the preference for pink flowers observed in the aviary did not translate into a female fitness advantage for this morphotype in the field, since no difference is found in natural pollination rate, fruit or seed set. This may be because the minimization of flight distances between plants is the primary factor in sunbird foraging choices, overriding their colour preference. Antagonistic nectar robbers did not act as a selective force on the polymorphism, since nectar-robbing rates were equal between white and pink morphotypes in the field.