In most hornbills the casque is a light structure: a keratin sheath over a cavity supported by a sparse network of bone. It sits above the upper mandible and is connected to the skull, and in the rhinoceros hornbill it curves upward at the front in the shape that gives the bird its name. Casque size and colour differ between the sexes and develop with age.
What the casque is for is not fully resolved. Proposals include amplification or resonance of calls, a signal of sex and maturity used in mate choice, and use in aerial casque-butting, which is recorded in some helmeted and other hornbill species. The evidence for resonance is weak, and signalling is the more commonly favoured explanation.
The exception matters. The helmeted hornbill, Rhinoplax vigil, has a dense solid casque of keratin known as hornbill ivory. Demand for carved casques has driven intense poaching, and that species is now Critically Endangered.