The most distinctive feature of the hornbills is the heavy bill supported by powerful neck muscles as well as by the fused vertebrae.The large bill assists in fighting, preening, and constructing the nest, as well as catching prey. A feature unique to the hornbills is the casque a hollow structure that runs along the upper mandible. In some species it is barely perceptible and appears to serve no function beyond reinforcing the bill. In other species it is quite large, is reinforced with bone and has openings between the hollow centre allowing it to serve as a resonator for calls. In the Helmeted Hornbill the casque is not hollow but is filled with ivory and is used as a battering ram used in dramatic aerial jousts.Aerial casque-butting has also been reported in the Great Hornbill.