Abstract:
Although the function of nectar is to attract and reward pollinators, secondary metabolites produced by plants as anti-herbivore defences are frequently present in floral nectars. Greater understanding is needed of the effects of secondary metabolites in nectar on the foraging behaviour and performance of pollinators, and on plant–pollinator interactions. We investigated how nectar-feeding birds, both specialist (white-bellied sunbirds Cinnyris talatala) and generalist (dark-capped bulbuls Pycnonotus tricolor and Cape white-eyes Zosterops virens), respond to artificial nectar containing the alkaloid nicotine, present in nectar of Nicotiana species. Preference tests were carried out with a range of nicotine concentrations (0.1–300 μM) in two sucrose concentrations (0.25 and 1 M), and for bulbuls also in two sugars (sucrose and hexose). In addition, we measured short-term feeding patterns in white-bellied sunbirds that were offered nicotine (0–50 μM) in 0.63 M sucrose. Both nicotine and sugar concentrations influenced the response of bird pollinators to nicotine. The birds showed dose-dependent responses to nicotine; and their tolerance of high nicotine concentrations was reduced on the dilute 0.25 M sucrose diet, on which they increased consumption to maintain energy intake. White-bellied sunbirds decreased both feeding frequency and feeding duration as the nicotine concentration in artificial nectar increased. Of the three species, bulbuls showed the highest tolerance for nicotine, and sugar type (sucrose or hexose) had no effect. The indifference of bulbuls to nicotine may be related to their primarily frugivorous diet. However, the response of white-eyes to nicotine in the dilute sucrose solution was very similar to that of sunbirds, even though white-eyes are generalist nectar-feeders. Additional testing of other avian nectarivores and different secondary metabolites is required to further elucidate whether generalist bird pollinators, which utilise dilute nectars in which secondary metabolites have stronger deterrent effects, are more tolerant of ‘toxic’ nectar.
