Population fluctuation in Sulawesi red-knobbed hornbills: Tracking figs in space and time

We studied spatial and temporal variation in a population of Sulawesi Red-knobbed Hornbills (Aceros cassidix) in relation to availability of fruit resources over a two-year period in the Tangkoko DuaSudara Nature Reserve, North Sulawesi, Indonesia. Fruit production did not show any discernable patterns over 22 months of study, in spite of the strong seasonality of rainfall. Figs were available in all months of the year, and fig biomass exceeded ripe nonfig-fruit biomass in 10 of 22 months. Hornbill densities fluctuated dramatically over time ((x) over bar = 51 birds . km(-2), range 9.3-82.7) and among habitats. Spatial and temporal variation in hornbill numbers was best explained by habitat selection and the abundance and distribution of figs. Hornbill numbers were higher in those areas with greater densities of fig trees, and monthly hornbill densities and mean nock size increased with increasing fig biomass. Because Sulawesi Red-knobbed Hornbills appear to track fig production over potentially large distances, and include a mix of other rainforest tree species in their diet, we hypothesize that they may play an important role as agents of rainforest regeneration.