Foraging characteristics of an assemblage of four Sumatran hornbill species

Abstract

This study examines the diets of four hornbill species (Bushy-crested Hornbill Anorrhinus galeritus, Wreathed Hornbill Aceros undulatus, Rhinoceros Hornbill Buceros rhinoceros and Helmeted Hornbill Buceros vigil) common within our 9 km2 study area in the Bukit Barisan Selatan National Park, Sumatra, Indonesia. Line transects and opportunistic sampling were used over one year to collect feeding data, including the tree species and strata in which birds fed, diet items, fruit characteristics and tree crop size. Hornbills fed on 64 species. Figs, non-fig fruits and animals comprised 23.4% (n = 15 species) 51.6% (n = 33 species) and 25% (n = 16 species) of the diet, respectively. Although there was overlap among the diets of the four species, only B. rhinoceros and B. vigil had a statistically significant overlap, primarily due to high fig consumption by both species. B. vigil fed almost exclusively on figs (98.6% of the diet) and a small proportion of animals (1.4%) while 23.1% of the B. rhinoceros diet comprised non-fig fruits and animals (23.1%), with 76.9% figs. A. galeritus and A. undulatus had distinct diets with drupaceous, oily fruits comprising 66.7% and 64.4% of the diets, respectively. We found no significant relationship between the numbers of hornbills visiting fruiting trees and the characteristics of the fruit (weight, length and width). However, maximum crop sizes of diet species significantly influenced the number of feeding hornbills. Hornbill species also differed in the strata in which they fed. A. galeritus and B. rhinoceros were seen in the middle of the canopy 56% and 50.8% of the time, respectively while A. undulatus and B. vigil were observed more frequently in the upper canopy (50% and 74.3% of the time, respectively). The lower part of the canopy was used infrequently by all species. This study suggests that the four hornbill species in our study partition food resources by food type and/or feeding location.