Pre-dispersal seed predation by a granivorous bird, the masked grosbeak (Eophona personata, Fringillidae), was investigated in two bird-dispersed trees, Celtis sinensis and Aphananthe aspera (Ulmaceae). The objectives of this study were to 1) measure direct damage of predation by grosbeaks on plant crops, 2) reveal the temporal pattern of predation within each tree species and its causal factors, and 3) test whether foraging grosbeaks hinder foraging of frugivorous birds, thereby indirectly impacting the reproduction of both tree species. A substantial amount of fruit and seed crop was consumed by grosbeaks (24.3% in Celtis; 55.5% in Aphananthe), and only 17.7% (Celtis) and 16.7% (Aphananthe) were removed by frugivorous birds. At the study site, the grosbeak population size fluctuated greatly during the fruiting seasons of both plant species. As for Celtis, predated seed density also fluctuated temporally, and the local population size of grosbeaks was responsible for predated seed density. In Aphananthe, predation was not fully explained by grosbeak populations or plant phenology, but its peak coincided with that of grosbeak population. These results suggest that predispersal seed predation by granivorous birds can have large negative impacts on the bird-dispersed plants. Changes in local population size of granivorous birds can influence predatation and can affect reproductive success of the bird-dispersed plants available to the birds.