Except for the aberrant Ground Hornbills, hornbills are canopy birds, feeding omnivorously on fruit, insects, reptiles and eggs of other birds, although the percentage of animal prey varies between species. Fruits are picked delicately, and then thrown in the air to be caught and swallowed. Hornbills typically choose smooth-skinned fruits (often red or purple ones) and swallow them whole, later regurgitating the stones for large-seeded fruits, or passing small seeds out in their faeces. In either case, they are important seed dispersal agents.
As birds of (mostly) primary rainforest, they are threatened by deforestation. This not only removes their food supply, but as often very large hole-nesting birds they are dependent on large emergent trees which are the only ones suitable for nesting.
The large bills of hornbills require special treatment in aviary birds. It is important the birds have appropriate forked sticks or ropes which they can use to clean their bills. If a birds bill is damaged vets have been successful in using dental acrylic or fibreglass to prepare prosthetic bills – although getting these to stay on considering the amount of use they get can be difficult.
The other big husbandry problem with hornbills is one shared by many other rainforest birds – a susceptibility to hemochromatosis or iron retention disease. Believed to be due to excess iron in the diet, this causes sudden death from liver failure, often in birds showing little signs of illness. The main reason for this disease is that rainforests often grow on soils extremely poor in minerals, with a corresponding lack of iron in the vegetation that grows on it. In addition the birds will ordinarily drink pure rainwater or water from tree hollows which is full of tannins which block iron absorption.. The tannins in the wild diet may be replaced by adding supplements to their water in captive birds – in this case in the form of cold tea!
Replacing the fruits eaten in the wild can also be problematic, for all fruit-eating animals, not just hornbills. Domestic fruits have all been bred over many thousands of years to appeal to human taste buds, and are typically much higher in water and sugar, and lower in fibre and protein, than wild fruits. Different wild fruits have very different nutrient compositions, with some producing fruits rich in fats, others rich in carbohydrates. Now the cause of hemochromatosis is understood, diets of captive birds are designed around low-iron pelleted foods, and citrus fruits are rarely fed. This last is because Vitamin C promotes iron uptake and so can be a health risk.
The most famous aspect of hornbill ecology is their strange breeding behaviour. At the start of the nesting season, the female is walled up in the chosen nest cavity by the male using mud, until only a narrow slit that the female can get her beak thjrough remains. This protects the female from predators, but means she is utterly dependent on the male for food for the several months it takes before the chicks are large enough to require two parents to feed them. In some species the female leaves before the chicks fledge and the chicks then re-seal the entrance behind her. When supplying hornbills with nestboxes, it is important that this behaviour is borne in mind and the right type of nestbox is supplied so this behaviour can occur, or the birds will not breed successfully.
For more on hornbills, see here: http://www.coraciiformestag.com/Research/Research.htm
(pictures from wikipedia)