Editing Hornbill (section)

===Breeding===
[[File:Britannica Hornbill Buceros bicornis.png|left|thumb|Male hornbill transfers a fig to the female.<ref>{{Cite EB1911|wstitle=Hornbill}}</ref>]]
[[File:Black-casqued hornbill male skeleton.jpg|thumb|Male black-casqued hornbill (''[[Black-casqued hornbill|Ceratogymna atrata]])'' on display at the [[Museum of Osteology]].]]
Hornbills generally form [[Monogamy in animals|monogamous]] pairs, although some species engage in [[cooperative breeding]]. The female lays up to six white eggs in existing holes or crevices, either in trees or rocks. The cavities are usually natural, but some species may nest in the abandoned nests of [[woodpecker]]s and [[Capitonidae|barbets]]. Nesting sites may be used in consecutive breeding seasons by the same pair. Before incubation, the females of all Bucerotinae—sometimes assisted by the male—begin to close the entrance to the nest cavity with a wall made of mud, droppings and fruit pulp. When the female is ready to lay her eggs, the entrance is just large enough for her to enter the nest, and after she has done so, the remaining opening is also all but sealed shut. There is only one narrow aperture, big enough for the male to transfer food to the mother and eventually the chicks. The function of this behaviour is apparently related to protecting the nesting site from rival hornbills.<ref>{{cite journal|title=Nest intruders, nest defence and foraging behaviour in the Black-and-white Casqued Hornbill ''Bycanistes subcylindricus''|journal=Ibis |year=1988|first=Jan|last=Kalina|volume=131|issue=4 |pages=567–571|doi= 10.1111/j.1474-919X.1989.tb04791.x}}</ref> The sealing can be done in just a few hours; at most it takes a few days. After the nest is sealed, the hornbill takes another five days to lay the first egg. [[Clutch (eggs)|Clutch]] size varies from one or two eggs in the larger species to up to eight eggs for the smaller species. During the incubation period the female undergoes a complete and simultaneous [[moult]]. It has been suggested that the darkness of the cavity triggers a hormone involved in moulting.<ref>{{cite journal|author=Stonor, C. R.|year=1937|title=On the attempted breeding of a pair of Trumpeter Hornbills (''Bycanistes buccinator'') in the gardens in 1936; together with some remarks on the physiology of the moult in the female|journal=Proceedings of the Zoological Society of London, Series A |volume=107 |issue=Part 3 |pages=89–94|doi=10.1111/j.1469-7998.1937.tb08502.x}}</ref> Non-breeding females and males go through a sequential moult.<ref>{{cite journal|author=Moreau, RE|year=1937|title=The comparative breeding biology of the African Hornbills (Bucerotidae)|url=http://www.kalro.org:8080/repository/bitstream/0/4920/1/REPRINTS%20COLLECTION%2036%20Split%202.pdf|journal=Proceedings of the Zoological Society of London, Series A|volume=107|issue=Part 3|pages=331–346|doi=10.1111/j.1096-3642.1937.tb00815.x|access-date=2018-03-06|archive-url=https://web.archive.org/web/20180306142227/http://www.kalro.org:8080/repository/bitstream/0/4920/1/REPRINTS%20COLLECTION%2036%20Split%202.pdf|archive-date=2018-03-06|url-status=dead}}</ref> When the chicks and the female are too big to fit in the nest, the mother breaks out the nest and both parents feed the chicks.<ref name=EoB /> In some species the mother rebuilds the wall, whereas in others the chicks rebuild the wall unaided. The ground hornbills do not adopt this behaviour, but are conventional cavity-nesters.<ref name=EoB />