Saturday, 14 June 2014

Hornbills 2: Malayan Black Honbill

Currently living next door to our Wrinkled Hornbill is  female Malayan Black Hornbill, Anthracoceros malayanus. A male is off show at the moment, but will hopefully join her shortly. They originate from South East Asia, which is a centre of diversity for the Bucerotiformes. Among the close relatives of hornbills are the hoopoes, kingfishers, rollers, and woodpeckers. Most of these were originally grouped together in the Coraciiformes, but these have now been split into separate orders.

These birds are rare in zoos worldwide, with most in Europe and only a few in the US. Very few are bred each year, even with established pairs (which ours are not). This is typical for hornbills even in the wild – many will not breed every year as the strain of raising even the typically small clutch of 1 or 2 is too much, and adults will take a break between years. As is also typical of animals with a low reproductive rate, hornbills are long lived birds, especially the larger species. The oldest on record is a female Great Hornbill Buceros bicornis who was believed to be over 50 when she died. Several others have been known to breed when in their 30’s.

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 nest box is supplied so this behaviour can occur, or the birds will not breed successfully. Sufficient experience has now been gained with captive breeding of hornbills that dimensions of nest boxes, both external and internal, are available. Another important feature is the shape of the entrance hole – ideally this is a vertical oval shape which at its widest point is just large enough to admit the female. This shape makes it easier for the pair to mud up the entrance at the start of the breeding cycle. Other features that need to be monitored carefully are the humidity of the nest cavity – as rainforest birds too low a humidity can seriously affect the health of the chicks. With the growth of camera technology, nests can now be monitored remotely to ensure everything is progressing properly as the chicks mature.

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