In his classic work The Hornbills published in 1995, Alan Kemp wrote "Trying to decide what other groups of birds are most closely related to hornbills is not quite so easy." Time has not made that decision any easier; the classification of these bizarre, large-billed birds is still debated. Most modern taxonomic treatments place hornbills within the order Coraciiformes together with their closest relatives: the hoopoe (Upupidae) and the woodhoopoes and scimitarbills (Phoeniculidae). This classification is based primarily on similarity in foot and jaw morphology, and a prolonged retention of quills by nestlings, which gives them a prickly "pin-cushion" look. All birds of these families nest in tree holes, but only hornbills seal the entrance to their cavities.
As of 2001, science recognizes 54 species of hornbills grouped within 14 genera and two subfamilies. All but two species are classified within the subfamily Bucerotinae. The exceptions are the terrestrial ground-hornbills, which fall within the subfamily Bucorvinae. The distinction between Bucerotinae and Bucorvinae is based on unique feather lice and anatomical and behavioral differences such as a greater number of neck vertebrae and the lack of nest-sealing behavior in the Bucorvinae. In 2001, S. Huebner and colleagues conducted detailed molecular studies of the two groups. They found that ground-hornbills were probably the earliest form.
All 54 hornbill species display unique anatomical features that clearly identify their affinities. These include being blessed with long, sweeping eyelashes on their upper lids and a fusion of the first two cervical vertebrae to provide support for large bills. All hornbills lack carotid arteries as well as the short feathers under the wings that cover the primary and secondary flight feathers of other birds. Finally, hornbills have unusual kidneys in that they are two-lobed instead of three, and the Z chromosome, one of a pair of sex chromosomes, is oversized.
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