Coraciiformes are generally recognized by the large and long bill, large head, short neck, short legs, and weak feet with short toes. The single feature shared by almost all families in the Coraciiformes is that the front three toes are fused partially at the base. The middle toe is fused to the inner toe at its base and to the outer toe for most of its length. This toe-fusion is a condition termed syndactyly and is the main criterion by which the order is defined. Coraciiformes also share, but are not unique, in the design of the palate bones (classi fied as desmognathous), in the lack of an ambiens muscle in the leg, and in the rather small feet.
Other features are widespread in the order but not present in or unique to each family. The wing has 10 primary feathers, often with a vestigial eleventh feather; the tail has 12 feathers in all families other than the motmots, hoopoes, and hornbills, for whom the tail has 10 feathers. The breast bone has two notches on the sternum in most families, but only one notch in common hoopoes (Upupa epops) and hornbills. The inner ear bone, or columella (stapes), is of a simple reptilian design in rollers and hornbills, but of a unique anvillike shape in hoopoes and woodhoopoes, and of a stirrup-like shape in kingfishers, bee-eaters, todies, motmots, and trogons. The eggs are generally white, rounded, and shiny, except for being oval in hoopoes and hornbills, and tinted light blue-green in hoopoes. The chicks hatch blind and with the upper mandible noticeably shorter than the lower. The chicks are naked in all families, except for hoopoes, which have patches of fine down. However, the chicks always develop later through a spiky "pin-cushion" stage, when their emerging feathers are retained in their quills for several days. Kingfishers and bee-eaters excavate and nest in earthen burrows, and their chicks have well-developed papillae on their "heel" joint, similar to those of honeyguides and woodpeckers (Pi-ciformes). Sexes are similar in most species, except for most hornbills and some kingfishers. Many species have a brilliant plumage, often with a large colorful bill, long tail, or tall crest. While there is no single character that is unique to and defines any combination of families that might comprise the Coraciiformes, there is an overlap in shared characters that links at least the ten main families as presented here.
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