As their name implies, the hornbills have a distinctive bony casque on the upper surface of their bills, which is partly or largely hollow. The function of this casque is still debated, as it is used to chisel off bark, as well as in combat between rival males, but a small opening to the mouth suggests that it may also serve as a resonance chamber, accounting for the distinctively nasal quality of the calls of several larger species. Other distinctive morphological features of these birds are long eyelashes, bare and often brightly-coloured skin around the eyes, and pronounced sexual dimorphism. But perhaps the most conspicuous characteristic of these magnificent birds, and often the only clue to their presence in the tall forest canopy, is the “whooshing” made by their wings when flying, which is due to their lack of underwing coverts that normally cover the base of the flight feathers. Contrary to popular belief, the hornbills are not related to the toucans, their ecological counterparts in the New World, but are instead related to the kingfishers and rollers, with which they share partially fused toes. They also nest in holes, but remarkably and uniquely, the incubating female seals herself into the nest cavity by plastering up the entrance hole with mud at first, then her viscid faeces, leaving only a small slit through which food, mostly fruit, can be passed by her mate. The imprisoned female then lays her eggs, usually two, and incubates them for 25 to 40 days depending on the size of the species, and often moults all of her flight feathers, becoming temporarily flightless. She breaks out of the nest towards the middle of the nestling period or when the young (usually only one) fledge, 50-90 days after hatching. After such a long period of confinement, she understandably becomes fat, dirty, and so stiff that she is barely capable of flying. This extraordinary reproductive strategy, however, presumably protects the nest occupants against predators such as monkeys, especially when the chicks are young and naked. Although 8-10 species of hornbills are found in Sumatra and Borneo, only three occur in Java, because of its smaller size; all three are found in Baluran
Noisy, with slow, deep wingbeats.