Evolution and systematics

The six families in the Piciformes order are: honeyguides (Indicatoridae), woodpeckers, wrynecks, and piculets (Pici-dae), barbets (Capitonidae), toucans (Ramphastidae), jacamars (Galbulidae), and puffbirds (Bucconidae)

The order Piciformes takes its name from the Roman forest god Picus who, according to myth, was turned into a woodpecker by the sorceress Circe for spurning her amorous advances. Like the eponymous Picus, most piciform birds are forest dwellers, and most share a particular adaptation to life in the trees: zygodactylous or "yoke-toed" feet, with two toes pointing forward and two toes pointing backward. This arrangement of digits helps piciform birds get a grip on rough bark while hopping along branches and up and down tree trunks.

Zygodactyly is not unique to piciforms, and in the eighteenth and early nineteenth century, taxonomists grouped various bird families together on the basis of this common foot structure plus other traits. Linneaus, for example, used the trait to group parrots and cuckoos with woodpeckers and toucans in the order Picae. Illiger (1811) also used foot structure as a factor when placing these four groups plus trogons, puffbirds, and jacamars in an order he called Scansores (from the Latin scansum, "to climb"). Marshall and Marshall (1871) similarly recognized an order Scansores, but placed toucans, barbets, cuckoos, and turacos in this category.

In 1953, Beecher (1953) proposed that barbets, jacamars, puffbirds, toucans, woodpeckers and honeyguides form a "natural unit," noting similarities in the jaw muscles, tongue, and other traits; many modern classification schemes retain this grouping of the six families.

Subsequent workers, however, have questioned the close grouping of jacamars and puffbirds with the other piciforms. In 1972, Sibley and Ahlquist presented evidence from electrophoretic protein analyses to suggest that jacamars and puffbirds were more closely allied to kingfishers (Coraciiformes) than to woodpeckers and their allies. After completing more sophisticated DNA hybridization studies, the same researchers reported in 1990 that the evidence supports grouping of woodpeckers with hon-eyguides and toucans with barbets (family Capitonidae) in the order Piciformes, but that jacamars (family Galbulidae) and puffbirds (family Bucconidae) should be placed in a separate order, the Galbuliformes. In 2001, Hofling et al. also suggested removing these two families from the Piciformes, but, based on the structure of the shoulder girdle, said that Galbulidae and Bucconidae more closely resemble the Coraciiformes than the Piciformes. This volume follows the convention of many contemporary taxonomies by placing all six families in the Pici-formes.

On the evidence of their DNA hybridization studies, Sib-ley and Ahlquist also conclude that piciform birds diverged

Green-barred woodpecker (Chrysoptilus melanochloros) in Goias, Brazil. (Photo by John S. Dunning. Bruce Coleman Inc. Reproduced by permission.)

from coraciiform birds in the Upper Cretaceous, 98 to 65 million years ago (mya). The oldest known fossil woodpecker bones, however, have been dated only to the Miocene, about 25 mya. A specimen of petrified wood collected in Arizona, dated to the Eocene (40-50 mya), includes a well-preserved woodpecker cavity and entrance hole.

Anatomical as well as genetic evidence supports the idea that jacamars and puffbirds differ from the other piciform families. For example, birds in former two groups have two carotid arteries, whereas members of the other four families have a single (left) carotid artery. Furthermore, jacamars and puffbirds have bare skin over the preen gland; most other piciform species have a feather-covered preen gland. Jacamars and puffbirds have an appendix; other piciformes lack an appendix. And in both jacamars and puffbirds, the syrinx is expanded and drumlike; however, this is not the case for the other families. Finally, jacamars and puffbirds are distinct from other piciforms in their nesting habits; whereas other piciforms make their nests or lay their eggs in tree cavities, these two groups most often breed in burrows that they excavate in soil.

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