Fruit removal by hornbills in a semi-evergreen forest of the Indian Eastern Himalaya

We tested the hypothesis that seed size influences which frugivores eat fruits, and the size and nature of disperser assemblages in Pakke Tiger Reserve, India. Four tree species had large seeds (> 18 mm width) that could be handled by birds with large gape widths, while two tree species had smaller seeds (< 7 mm width) falling within the gape size range of many frugivores. We tested whether (1) disperser assemblage and activity reflected seed size, and (2) large-gaped hornbills were more effective at fruit removal of tree species with large seeds, than of those with smaller seeds and many dispersers. Day-long watches were conducted in 2005 at trees of Dysoxylum binectariferum (three in 2005 and nine in 2006), Chisocheton cumingianus (nine), Aglaia spectabilis (seven), Polyalthia simiarum (nine), Litsea monopetala (four) and Cinnamomum bejolghota (two in 2005 and six in 2006) to determine which frugivores visited trees and ate fruit. Disperser visitation and species diversity per tree to species with medium-sized seeds averaged 85 visits and 10 species d(-1), contrasted with five visits by one frugivore species to large-seeded tree species. Seed removal rates per tree averaged 486 seeds d-1 from medium-seeded tree species, but 10 seeds d-1 from large-seeded species. Hornbills (Bucerotidae) and Ducula badia (Columbidae) removed large seeds from capsules of Aglaia spectabilis, Chisocheton cumingianus and Dysoxylum binectariferum (Meliaceae). Primates, civets and bats also consumed drupes of large-seeded Polyalthia simiarum (Annonaceae). Anthracoceros albirostris hornbills were important dispersers of Litsea monopetala (Lauraceae), a medium-seeded tree with a large disperser assemblage. Conversely, hornbills were quantitatively inconsequential for Cinnamomum bejolghota (Lauraceae), another medium-seeded tree species with several dispersers. Results suggest that the size and activity of disperser assemblages accurately reflects seed size. While hornbills were quantitatively important dispersers of large-seeded tree species, their effectiveness for trees with small-to medium-sized seeds depended on the tree species.