Hornbills are among the most flamboyant birds of their habitat. The oversized, slightly decurved bills topped by sometimes outlandish casques shaped as bumps, ridges, or horns make hornbills an unforgettable component of any landscape. Hornbills vary tremendously in size and shape, starting with the large, long-legged southern ground-hornbill (Bucorvus leadbeateri) weighing up to 13.2 lb (6 kg), and going down to the 0.26 lb (120 g) red-billed dwarf hornbill (Tockus camurus). Males are always larger and stouter than females but the greatest dimorphism often occurs in bill length with males having up to 30% longer bills. Hornbill plumage is described as "drab," lacking the brilliant colors of relatives such as the kingfishers (Alcedinidae) and rollers (Coraciidae). However, the bold black-and-white patterns of many forest hornbills and the delicate gray pied patterns of many Tockus species are far from dull. Add in bills and casques of brilliant orange, yellow-gold, deep crimson, or shiny black, and patches of bare skin around the eyes and throat in a kaleidoscope of garish hues, and you have a colorful group of birds.
Plumage color and size and shape of the casque identify the age and sex of an individual. Newly fledged hornbills have
underdeveloped casques and small bills, but after the first year of life, appearances converge on that of their adult counterparts. In species where sexes differ in color as adults, determining the gender of the young can be difficult. For example, in almost all Aceros, Rhyticeros, Penelopides, and Tockus species, the young, regardless of their sex, resemble their fathers for the first year of life. The opposite is true for the Bycanistes and Ceratogymna who resemble the adult female. Young of the northern ground-hornbill (Bucorvus abyssinicus) and a few Tockus species show plumage true to their sex while chicks of the rufous hornbill (Buceros hydrocorax) are radically different from both parents.
Numerous authors have described the noise produced by flying hornbills as that of an approaching train. This incredible "whooshing," produced in different pitches depending on the species' size, is a result of wing structure. Because hornbills lack the small feathers that normally cover the shafts of the primary and second flight feathers, each powerful stroke of the wing allows air to pass through and vibrate the large feathers.
The most outstanding feature, and the one from which hornbills acquire their common name, is the casque on the top of the bill. Casques vary from the mere ridge of the red-billed hornbill (Tockus erythrorhynchus) to the wash-board bumps of the wreathed hornbill (Rhyticeros undulatus) and the elaborate banana of the rhinoceros hornbill (Buceros rhinoceros). The function of casques, which may take up to six years to develop, is the topic of many debates. It is possible that casques provide structural support for a long bill. Casques may also serve an acoustic function by helping amplify a horn-bill's call. Additionally, casques may be attractive to the opposite sex. The helmeted hornbill uses its casque in bizarre, aerial displays where individuals of either sex collide in midair, casque-to-casque. The head-butting competitions always occur near fruiting fig trees (Ficus spp.). Although Gustav Schneider once reported that helmeted hornbills perform this comical ritual when they are intoxicated on fermented figs, observations from Sumatra indicate that this acrobatic act may be in defense of clumped food resources.
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