BIRD ACTIVITY AND SEED DISPERSAL OF A TROPICAL WET FOREST TREE
Feeding assemblages of birds were observed throughout a fruiting season at a Costa Rican population of the rain forest tree Casearia corymbosa in order to distinguish effective dispersal agents from visitors which used arils for food without dispersing seeds. This system is of special interest because the tree fruits during an annual period of fruit scarcity in a forest characterized by high proportions of animal-dispersed plants and obligate frugivores. Results show that the Masked Tityra (Tityra semifasciata) is an effective dispersal agent of seeds of this tree because it: (1) regurgitates viable seeds, (2) is a common and regular visitor throughout the season, (3) has high feeding rates, (4) removes seeds from the vicinity of the parent tree before processing them, and (5) depends on this rather than other plants fruiting at the same time. Twenty-one other visitors were deficient dispersers, although some depended heavily on fruit of this tree for food. Two parrots (Amazona autumnalis and Amazona farinosa) preclude dispersal by strip- ping arils with their bills and dropping all seeds in situ. Fourteen species are occasional visitors. Five visitors (Ramphastos sulfuratus, Ramphastos swainsonii, Pteroglossus torquatus, Myiozetetes similis, and Myiozetetes granadensis) process seeds undamaged and are sometimes numerically abun- dant, but are absent part of the season and tend to regurgitate seeds in situ. Because fruiting activity of this tree occurs during annual scarcity, it represents a pivotal species in the community. Three obligate frugivores, including one effective disperser and two fruit thieves, depend almost entirely on it for food for periods of 2-6 wk. I hypothesize that extinction of this plant at this site would lead to disappearance of these birds, and would almost certainly depress recruitment of other species of trees for which they serve as dispersal agents at other times of the year.