Uganda ToursJuly 21, 2023
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Female great hornbill feeding on figs. Fruit forms a large part of the diet of forest hornbills.
Hornbills are omnivorous birds, eating fruit, insects and small animals. They cannot swallow food caught at the tip of the beak as their tongues are too short to manipulate it, so they toss it back to the throat with a jerk of the head. While both open country and forest species are omnivorous, species that specialise in feeding on fruit are generally found in forests, while the more carnivorous species are found in open country. Forest-dwelling species of hornbills are considered to be important seed dispersers. Some hornbill species (e.g Malabar pied-hornbill) even have a great preference for the fruits of the strychnine tree (Strychnos nux-vomica), which contain the potent poison strychnine. 
Some hornbills defend a fixed territory. Territoriality is related to diet; fruit sources are often patchily distributed and require long-distance travel to find. Thus, species that specialise in fruit are less territorial.
Male hornbill transfers a fig to the female.
Male black-casqued hornbill (Ceratogymna atrata) on display at the Museum of Osteology.
Hornbills generally form 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 woodpeckers and 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. 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 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. Non-breeding females and males go through a sequential moult. 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. 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.
Associations with other species
A number of hornbills have associations with other animal species. For example, some species of hornbills in Africa have a mutualistic relationship with dwarf mongooses, foraging together and warning each other of nearby birds of prey and other predators. Other relationships are commensal, for example following monkeys or other animals and eating the insects flushed up by them.