Hornbills (Bucerotidae) are a family of bird found in tropical and subtropical Africa, Asia and Melanesia. They are characterized by a long, down-curved bill which is frequently brightly colored and sometimes has a casque on the upper mandible. Both the common English and the scientific name of the family refer to the shape of the bill, "buceros" being "cow horn" in Greek. In addition, they possess a two-lobed kidney. Hornbills are the only birds in which the first two neck vertebrae (the axis and atlas) are fused together; this probably provides a more stable platform for carrying the bill. The family is omnivorous, feeding on fruit and small animals. They are monogamous breeders nesting in natural cavities in trees and sometimes cliffs. A number of species of hornbill are threatened with extinction, mostly insular species with small ranges.