They are found all over the world, from sub-Saharan Africa to India, Malaysia, New Guinea and the Solomon Islands. Hornbills also vary greatly in size, with the largest, the Southern ground hornbill, weighing in at 4kg, and the smallest, the Upper Guinea dwarf black hornbill, only tipping the scales at 109g.
Males are the same size or slightly larger than the females.
Although there is such a wide range of hornbills, there are many things that are common to most, if not all the species. All have the horn casque on the upper mandible, which is a horny layer of keratin, containing little or no bone structure. This varies in size and density from species to species, and only develops as the birds grow older - young birds simply have a pale area well supplied with blood vessels, from which the casque will develop. Scientists are not absolutely sure of the purpose of the casque. Some postulate that it evolved to provide support for the long curved bill, which is subject to much stress as hornbills have extremely strong jaws.