Seed dispersal effectiveness by two bulbuls on Maesa lanceolata, an African montane forest tree

We examined the effectiveness with which two common understory birds, Andropadus latirostris and A. tephrolaemus (Pycnonotidae), dispersed seeds of a tropical pioneer forest tree, Maesa lanceo(ata (Myrsinaceae). Relative dispersal effectiveness of each species was assessed in terms of quantity and quality of seed dispersal. The number of seeds dispersed was examined by quantifying the tree visitation frequency, duration of time in the tree and number of fruits ingested, and probability that the bird would deposit seeds under the parent tree. The quality of seed dispersal was examined using defecation frequency, number of seeds defecated through time following ingestion of a cohort of 20 fruits, seed viability after passage through the gut, and general diet analysis. A,ndropadus tephrolaemus and A. latirostris were the most common visitors to M. lanreolata and accounted for 49.1 percent and 17.3 percent of all animal visits to the tree, respectively. The mean (and standard deviation) residence times in the tree were 2.82 +/- 1.65 min for A. tephrolaemus and 3.14 +/- 1.70 min for A. latirostris. Individuals of both species are estimated 23 to 25 fruits per visit (3-20 seeds/fruit). Mean minimum seed passage times were 17.8 +/- 1.93 min for A. tephrolaemus and 12.9 +/- 0.80 min for A. latirostris. Seeds were viable after passing through the birds' gut. The majority of seeds passed through the gut within 20 min of the first defecation that contained seeds, and nearly all seeds passed within 80 min of ingestion. Mean time between defecations was 2.6 min for A. tephrolaemus and 2.2 min for A. latirostris. Frequency of return to the tree combined with the short tree residence rimes and defecation rates suggest that both species are unlikely to defecate more than 5 percent or 10 percent of seeds under the parent tree. Both species consistently dispersed most seeds away from the parent tree; A. tephrolaemus may be a more important disperser for the tree, however, because of its higher visitation rate.