THE ROLE OF AN AVIAN NECTAR ROBBER AND OF HUMMINGBIRD POLLINATORS IN THE REPRODUCTION OF 2 PLANT-SPECIES

1. Flower nectar robbers which extract flower rewards to pollinators b ut do not pollinate, may reduce the fitness of the plant. In this stud y we combined field observations and experimental manipulations to ass ess the role of a primary nectar robber, the bird Diglossa baritula, o n flower nectar secretion and reproductive output of two hummingbird-p ollinated plant species with contrasting breeding systems. We used the hermaphroditic Salvia mexicana and the androdioecious Fuchsia microph ylla. In addition, because the distinction between pollinators and rob bers is not so sharp, we compared the pollination efficiencies of D. b aritula and of five species of humming birds visiting the flowers of t he same plant species. 2. Flowers of the two plant species were freque ntly robbed (92 and 37% of the flowers in Salvia mexicana and Fuchsia microphylla respectively). For both species, field censuses of robbed and unrobbed flowers showed no differences in fruit set. Experimentall y induced nectar robbery did not affect the cumulative production of n ectar in both plant species. However, in S. mexicana, but not in F. mi crophylla, intact flowers produced more concentrated nectar. 3. The ro le of D. baritula and hummingbirds on seed production per flower, was assessed by exposing individual flowers to different schedules of visi tation by caged flower visitors. In S. mexicana, the number of seeds p roduced by flowers visited by the robber only was similar to that of f lowers visited by the least efficient pollinator but lower than that o f those visited by the other pollinators. 4. In F. microphylla, seed p roduction by flowers visited by the robber only was lower than that of flowers visited by all other pollinators. 5. For both plant species v isitation by the robber plus hummingbirds yielded a similar number of seeds as flowers visited by the more effective pollinator. 6. There wa s no field evidence that nectar robbery by D. baritula damages nectari es or ovules and, since under natural conditions flower visitation by the robber only was very rare, we conclude that for these two plant sp ecies D. baritula may be regarded as a commensal or even a low-efficie ncy pollinator.