We conducted experimental manipulations and field observations to determine the role of members of a nectarivorous guild (pollinators and robbers) on nectar production and pollen deposition in Hamelia patens at the La Selva Biological Station (Costa Rica). Seven pollinators (hummingbirds) and four robbers (1 hummingbird and 3 perching birds) comprised the avian nectarivorous guild visiting H. patens during March 1997. In addition, two florivorous birds were observed visiting H. patens during the study. Pollinators accounted for 85.6 percent of the visits, robbers for 12.4 percent, and florivores for 2 percent of the visits. Visitation by pollinators and robbers was greatest when floral nectar was highest. No aggressive interactions between pollinators and robbers were observed during the study. Pollinators differed in their ability to carry and deposit pollen on the stigma. Territorial hummingbirds were the least effective pollinators but the most frequent visitors. Flowers were frequently robbed (71%) during the study. Flowers experimentally robbed did not increase nectar production compared to control flowers; therefore, robbery may not involve an extra energy investment in terms of nectar production. The number of pollen grains deposited on artificially robbed flowers was significantly less than the number found in flowers with extra nectar (nectar added) but did not differ from the number found in non-manipulated flowers, indicating that nectar robbers may not affect the foraging behavior of hummingbirds, and therefore pollen deposition.
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Vol. 35 • No. 1