Fruiting phenology of fleshy-fruited plants and seasonal dynamics of frugivorous birds in four vegetation zones on Mt. Kinabalu, Borneo

An altitudinal survey of correspondences between fruiting phenologies of fleshy-fruited tree species and seasonal dynamics of frugivorous birds on Mt. Kinabalu in Borneo was carried out for 50 weeks across four vegetation types: a hill forest (800 m asl), a lower montane forest (1700 m), an upper montane forest (2000-3000 m) and a subalpine forest (3000-3500 m). In the hill forest, a large fruiting peak following the general flowering phenomenon was observed during October-November in 1996 and a fruitless period was observed during February-April in 1997. During the fruitless period, the number of resident frugivorous birds decreased. A bimodal fruiting pattern was observed in the lower montane forest. A large number of frugivorous temperate migrants were observed when the fruiting peak occurred. The number of resident frugivorous birds increased and several lowland bird species were observed, when the number of resident birds decreased in the hill forest. In the upper montane forest and the subalpine forest, more continuous and irregular fruiting patterns without outstanding peaks were observed and the number of resident frugivorous birds was more stable throughout the year. These suggested (1) there was a strong relationship between fruiting seasonality and seasonal dynamics of temperate migrants in the lower montane forest; (2) seasonal altitudinal movements of lowland bird species to montane vegetation might occur during the fruitless period in the lowland forest; and (3) the continuous fruiting pattern in the higher vegetation zones might be related to the scarcity of available frugivorous birds. The hypothesis that the influx of temperate migrants into the montane vegetation of Mt. Kinabalu is affected by density and habits of resident frugivorous birds is supported. Montane vegetation in Borneo plays an important role as temporal refugia for temperate and altitudinal migrants by supplying fruit resources.