Seed retention time (SRT) of 16 fruit species in the guts of the Brown-eared Bulbul (Hypsipetes amaurotis), a major fruit consumer in central Japan, was studied to examine the relationship between SRT and fruit characteristics, i.e. fruit size, seed size, seed weight, and water content. Caged bulbuls were videotaped after feeding on fruits, and the time of defecation of each seed was recorded. Most seeds were always defecated in fecal pellets, with the exception of Aucuba japonica (the largest of the seeds studied), a seed of which was regurgitated on one occasion. Bulbuls defecate large seeds more rapidly than small seeds. The SRT of the last defecated seed, mean SRT, and standard deviation of SRT were significantly negatively correlated with seed size, fruit size, and seed weight, while SRT of the first defecated seed and water content were not correlated with any of the fruit characteristics examined. This suggests that Brown-eared Bulbuls are somehow able selectively to eliminate bulky seeds from the gut rapidly in order to overcome digestive limitations. If birds would prefer fruit species with large seeds that they can regurgitate and with short seed retention times in the gut, the results suggest that large seeds have the advantage of quantity of seed dispersed. Small seeds retained in the gut for longer have the advantage of being carried further and thus can achieve greater dispersal distances and more diverse destinations. The evolutionary interaction between fruiting plants and avian seed dispersers, may affect the diversity of fruit characteristics mediated by the length of retention time in a bird's gut.