Effectiveness of three turacos as seed dispersers in a tropical montane forest.

We studied the effectiveness of three species of turacos, the Great Blue Turaco, the Ruwenzori Turaco, and the Black-billed Turaco, as seed dispersers in a tropical forest in Rwanda. For each species of turaco, we examined two factors affecting the effectiveness of seed dispersal: (1) the proportion of ingested seeds dispersed away from the parent tree and (2) the distances seeds were dispersed. To estimate these measures of effectiveness, we systematically observed the foraging activities of focal birds over a 14-month period. We also fed six species of tree fruits to captive Ruwenzori Turacos to determine the gut retention times for seeds of various sizes. Based on these measures, we calculated the seed shadows generated by each turaco species for tree seeds with either long or short gut retention times. The gut retention time of seeds was not correlated with seed size, but was positively correlated with the time interval over which seeds were defecated. Seeds with a long gut retention time were not only dispersed farther away from the parent tree, but were also deposited over a longer time period and were thus likely to be dispersed over a greater variety of habitats. Of the three turacos, the Ruwenzori Turaco deposited the highest percentage of ingested seeds away from the parent tree because it had the shortest residence time in feeding trees. However, the Ruwenzori Turaco also dispersed seeds for the shortest distance, partly due to its short flight distances. The Great Blue Turaco on average dispersed seeds the farthest due to its long flight distance and long gut retention time. The Black-billed Turaco, which fed least frequently, deposited seeds more evenly among perching sites than did the other two species. All three species of turacos dispersed over 80% of ingested seeds away from the parent tree.