Saturday, 7 June 2014

Hornbills 1: Wrinkled Hornbill

Scattered around the zoo we have several pairs of various species of hornbills, the distinctive, medium-sized to large birds that are among the most recognisable of forest birds in the Old World tropics. With around 55 species currently recognised (although some of the island species in Asia may be split), the hornbills fall into two natural groups. One contains the gigantic, terrestrial-feeding ground hornbills Bucorvus and their close relatives the Trumpeter hornbills in Bycanistes, which are grouped in the Bucorvinae. The other subfamily is the Bucerotinae, which includes all the other species. Bucorvines are restricted to Africa, while the Bucerotines are found in both Africa and Asia. Sadly, we do not have any ground hornbills at Bristol, but they are reasonably common in zoos around the world.

The most distinctive feature of the hornbills is the casque, a large, air filled development of the upper mandible. In most species the casque is larger in the male, but how large it is depends on the species. This large bill often means that hornbills are confused with toucans, which have a similarly over-sized bill. Toucans are not closely related however, and are only found in the New World tropics, where they have a similar diet, size, and lifestyle and can be thought as hornbill analogues.

In the wild, various hornbill species can be found in a wide variety of habitats, from dense primary rainforest to savannah woodland in dry or highly seasonal climates. As a group they are omnivorous, with a diet centred on forest fruits supplemented with insects and small vertebrates. The proportion of animal protein varies considerably depending on habitat, with terrestrial feeding or open-country forms taking more insects while the rainforest species feeding heavily on fruit, especially figs.
The picture at the head of this post is of our pair of Wrinkled Hornbills, Aceros corrugatus. Originally from the Malay peninsula, extending to Sumatra and Borneo, they are birds of primary lowland rainforest up to 1000m. They will sometimes use selectively logged forest that has regenerated after logging, but seem to require primary forest for breeding. Unfortunately this is also the forest that is the main target of the logging industry, and as they seem to be quite thinly distributed at the best of times the fragmentation of the surviving forest must be having a serious effect. It is currently listed as Near Threatened, but the actual situation may be more serious than that.

As with all hornbills except the Ground Hornbills, the Wrinkled Hornbill has a unique breeding strategy in which the female is walled up inside the nest cavity with a mixture of mud and faeces while the eggs are incubated. The male feeds the female who then feeds the young fledglings until the chicks are well grown, at which point the mud is removed and the female (who has undergone a complete moult while inside) can then aid the male in feeding the chicks.
The pair bond in hornbills is very strong – all known species are monogamous and pairs can remain together for many years. Lifespan can be very long – some of the larger species have lifespans in captivity of over 60 years. In some cases cooperative breeding has been observed, where additional birds (probably young from previous years) aid in feeding the chicks. This has not been reported in Wrinkled Hornbills, but at least one other species of Aceros is known to use this strategy.

Our own pair of Wrinkled Hornbills is still young, only a few years old, so we do not expect any breeding attempts for at least another year or two. They seem to have settled in well, and the pair appears to be compatible, which is a good omen for the future.
Next time, I will talk about the other large hornbill we have at Bristol, the large and impressive Malayan Black Hornbill.

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