Colours of Wildlife - Silvery-Cheeked Hornbill

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Silvery-Cheeked Hornbill

Willem is a wildlife artist based in South Africa. He says "My aim is simply to express the beauty and wonder that is in Nature, and to heighten people's appreciation of plants, animals and the wilderness. Not everything I paint is African! Though I've never been there, I'm also fascinated by Asia and I've done paintings of Asian rhinos and birds as well. I may in future do some of European, Australian and American species too. I'm fascinated by wild things from all over the world! I mainly paint in watercolours. . . but actually many media including 'digital' paintings with the computer!"

Silvery-cheeked hornbill by Willem

One can never have too many hornbills! This is the Silvery-cheeked Hornbill, Bycanistes brevis, sometimes placed in the genus Ceratogymna. This one I haven’t seen yet. It is the largest species found in South Africa apart from the Southern Ground Hornbill, reaching 80 cm/32" in length and about 1.4 kg/3 lbs in weight. It is only very occasionally seen here, in the far northeast of the country. It is more abundant in Zimbabwe, and from there its distribution stretches northward into Mozambique, Malawi, Tanzania, Kenya and Ethiopia. It is named for the light-tipped feathers around its cheek region. There are two other 'cheeky' hornbills in Africa: the Brown-cheeked and the Grey-cheeked.

The Silvery-cheeked Hornbill is one of a group of mainly black-and-white hornbills found in Africa, that have huge casques on the tops of their bills. In South Africa the Trumpeter Hornbill, Bycanistes bucinator, is similar but slightly smaller, with a smaller casque as well. The cream-coloured casque of the Silverycheeked Hornbill is so huge that it makes its head look bigger than its body! These birds look very funny while flying: they stick out their necks with their huge heads held out front, and the little body and wings trailing along behind it. They look completely unbalanced. But actually the casque is very light – it is mostly hollow. Its outer surface is made of keratin, the same substance as its beak – and in us humans, our fingernails. Inside it has a few bony struts towards the rear to give it extra support. There is a considerable difference between the sexes in this species: the female has a much shorter casque, only extending about half way to the tip of her bill, and it is much lower than that of the male as well, and the same horny colour as the rest of her bill.

Science still does not fully understand the function of the casque. Being very conspicuous, it probably has a significant display function. Baby birds have virtually no casque; it starts developing in youngsters. Subadult male and female birds have casques similar to that of the adult female, but only adult males develop the full-sized creamy-yellow casque. The casque is broad at the back but tapers to a narrow tip. So, in them the casque shows their sex as well as their maturity. Also it is biggest in the males that are the 'manliest', that is to say, that have the highest levels of testosterone, as well as being strong, healthy and vigorous. So the female could pick out the best mate to a large extent simply by comparing the casques of the contenders.

Another possible use of the casque is as a resonating chamber. Silvery-cheeked hornbills have loud, braying calls that carry far in the forest. (They also have softer calls, growling, grunting, hooting and quacking, that they use to stay in touch with flock members while foraging). Seeing as the casque is mostly hollow, the volume of air inside it can be set in motion and resonate in feedback with the bird's vocal chords, or augment certain frequencies of the call. But exactly how this works has not yet been adequately studied.

These hornbills live mostly in pairs or as trios – the male and female along with an immature offspring. They live in well-developed evergreen forest, mostly in mountainous regions, but in a few places frequenting lowland, coastal and riverine forest, and sometimes venturing into deciduous woodland. They are fruit eaters, being especially partial to figs, and at large fruiting trees might form larger flocks, of up to 100 birds. They can also roost communally in flocks of up to 200 birds, but they disperse from there when setting out to feed early in the morning. An interesting behavior of theirs is sun bathing: just before setting out they will often spread out their wings and let their heads loll about while catching some of the warm early-morning rays. They also often dust-bathe on the forest floor. While mostly picking fruit directly off the branches, they will also sometimes pick up ripe fruit from the ground. They sometimes catch insects and may take advantage of eruptions of locusts. They might also catch and eat small animals like lizards and even baby birds of other species.

In eating fruit these birds play a role in dispersing their seeds. They swallow even quite large fruits whole. If these fruits have large seeds, these are regurgitated, while smaller seeds are passed in their droppings. In both cases this helps the seeds to germinate more easily. Most edible fruits have a germination inhibitor in the pulp, so that the seeds only germinate if this is removed. So when a bird eats it and digests the pulp, but defecates or regurgitates the seed – which would be some distance from the parent plant, hopefully – it is ready to germinate. If passed in the droppings, it is assisted by a helpful dollop of fertilizer as well.

In the case of mistletoe, though, the seeds are surrounded by a very sticky flesh, and the only way to get rid of the seed is to actively wipe it off against a branch. But this suits the mistletoe as well, since it is a parasite which grows on the twigs and branches of other plants. After being rubbed off the seed will stick there, germinate and send a root into the tissues of the branch, from where it will steal the host tree's nutritious juices and grow into a small shrub up in the canopy of its host. Usually the host will not be killed, merely inconvenienced. There is a wide variety of mistletoes and similar parasitic plant species in Africa, many of them quite interesting and with attractive flowers, and they, too, are very dependent on fruit-eating birds like hornbills for their reproduction and propagation.

The reproduction of this species is quite similar to that of other hornbills, as described in my article on the Monteiro's Hornbill. In these, too, the female incarcerates herself in a big tree hole, or rarely a hole in a rock face, and the male brings food for her and, later, her chicks. Silvery-cheeked hornbills wall up the tree hole with mud: they first swallow the mud, and in their crops they form this into sticky pellets bound together with saliva, which they regurgitate again and use as 'bricks' for building the nest wall. The male is the one who goes searching for suitable mud, then swallows it and brings it to the female. During the breeding period the male will also defend the female and the nest, being very aggressive especially against other male silvery-cheeked hornbills. Once she's safely inside his big job is finding food. He has a crop that can distend a lot so that he can gather and store lots of fruit which he then can bring to her. Once the chicks are hatched he will bring food to her and them up to 20 times over the course of each day.

These hornbills mostly occur in small, disconnected patches of forests, and are quite dependent on our conservation of this habitat for their continued survival.

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