Piciformes

(Woodpeckers and relatives)

Class Aves

Order Piciformes

Number of families 6

Number of genera, species 62 genera; 383 species

Evolution and systematics

The six families in the Piciformes order are: honeyguides (Indicatoridae), woodpeckers, wrynecks, and piculets (Picidae), 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 honeyguides 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 Piciformes.

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

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.

Physical characteristics

Piciformes are small to medium-sized birds of forests and woodlands. In addition to having distinctive "X"–shaped feet, members of this group share other skeletal features: they have 14 cervical vertebrae; all of the thoracic vertebrae are unattached; the sternum has four (two pairs of) notches; and they have five complete ribs.

Piciformes share other common features in their musculature, digestive system, and plumage. For example, many members of this group have stout, sturdy beaks, which they use for gouging wood or other substrates to obtain food. Also, in most species the adult birds lack down feathers (jacamars are an exception to this pattern). And though plumage patterns and colors are quite varied for the group as a whole, combinations of black and white with accents of red and yellow are common, and males and females of a species often look alike. Woodpeckers (one of three groups in the family Picidae) are unusual in this family and indeed among all birds in having extra-stiff tail feathers, which they use to brace their bodies against tree trunks while climbing vertically or hammering with their beaks. Barbets do use their tails as a brace, but only while excavating the nest cavity.

Distribution

Most piciforms are year-round residents in their home ranges and do not migrate. Just the same, this is one of the most widespread avian orders, mostly thanks to the woodpeckers, which are represented on five continents (they are absent only from Australia and Antarctica). Members of the five other piciform families are less widely distributed; toucans, jacamars, and puffbirds are restricted to the New World tropics, and honeyguides occur only in Africa and southeast Asia. Barbets are found both in both the New World and Old World tropics.

Habitat

Almost all birds in this group are tree-dwellers, and for many species, the preferred habitat is mature forest with a closed canopy. Most piciform species are rarely, if ever, seen walking on the ground or flying across open space; typically these birds search for food in trees, nest in trees, raise young in trees, and roost at night in trees (usually in cavities). Of course there are exceptions to the rule of an arboreal life; for example toco toucans (Ramphastos toco) live in open, fragmented forests and woodland savannas. And many kinds of puffbirds and jacamars prefer forest edges and streambanks over forest interiors.

Behavior

Most piciform birds are not particularly social. Breeding is usually solitary, not colonial. In many species, however, male and female maintain the pair-bond and defend a shared territory year-round; this behavior is especially common in woodpeckers, puffbirds, and barbets. Some species of toucans do form small, loose flocks when foraging, and barbets and honeyguides may congregate temporarily where food is plentiful.

Though they are good climbers, many species of piciform birds are described as "weak" flyers. (Acrobatic honeyguides are one noteworthy exception.) Most members of this taxon do not migrate, although a few species (notably the yellow-bellied sapsucker [Sphyrapicus varius]) migrate long distances between breeding and wintering grounds.

Two behaviors exhibited by some birds in this taxon are extremely unusual in the avian world. One is drumming; woodpeckers routinely communicate in this distinctive way, hammering rhythmically, in species-specific patterns, on resonating structures such as hollow trees. A few species of barbet also communicate by drumming. Another is guiding, the behavior for which the honeyguides are named. Two honeyguide species in Africa "guide" other animals—including honey-badgers, baboons, and humans—to bees' nests. The birds alert their foraging partners to the presence of a honey-loaded hive with their calls and make short flights to indicate the direction of travel. If one of these larger animals does locate and break open the hive, the birds dart in to feast on beeswax.

Feeding ecology and diet

Feeding habits vary within this group; woodpeckers, jacamars, and puffbirds eat mainly insects, whereas toucans and barbets feed mainly on fruit (although these birds do feed their nestlings insects and similar protein-rich prey). Toucans are often described as active predators on eggs and nestlings of other birds but Remsen et al. (1993) contest this characterization.

Both jacamars and puffbirds take their insect prey in flight, behaving like oversized flycatchers. Butterflies are the preferred prey for jacamars; puffbirds most often capture flying beetles. Some woodpeckers also "hawk" after insects, but most often members of this group drill holes in tree bark to extract soft insect larvae (a few barbet species also excavate for insects). Some woodpeckers do eat fruit or nuts, and the appropriately named sapsuckers drill "sap wells" in trees and drink the sticky exudate.

Honeyguides are unique among birds for their habit of eating mainly beeswax from honeycombs; a symbiotic microorganism living in the gut helps these birds to extract nutrition from a substance that most other animals find indigestible. Honeyguides are also unusual in having an excellent sense of smell; many reports exist relating how birds are attracted to burning beeswax candles in rural churches.

Reproductive behavior

Sexual dichromatism is uncommon in piciforms. Most often, males and females look alike, probably because birds that maintain a year-round monogamous pair bond do not require elaborate courtship displays. In woodpeckers, though males and females often have different plumages, the differences between the sexes tend to be subtle, involving the color of nape patches

or the presence or absence of "moustaches." Neotropical barbets are the exception to the rule of uniform plumage for this group; all barbet species show marked differences between males and females with regard to plumage color and/or pattern.

Most piciform birds are cavity-nesters; even the honeyguides, all of which are nest parasites, lay their eggs in the nests of other hole-nesting species such as barbets and woodpeckers. The type of cavity used varies among families. Some species of jacamars and puffbirds dig out nest sites in rotten trees where termites have nested. Other species in these two families excavate their nesting burrows in soil, often along riverbanks. Barbets and woodpeckers use their strong, sharp beaks to hammer out nest cavities in rotting trees, and the largest toucan species occupy natural tree cavities. The smaller toucan species often drive woodpeckers away from just-excavated holes, then use their powerful beaks to enlarge the nest opening.

Almost all members of this group lay white eggs. Unpigmented eggs are typical of cavity-nesting birds—with the nest hidden from predators, there is no need for the eggs to be camouflaged.

Helping at the nest, an uncommon bird behavior, is often seen in woodpeckers, and is also known in some species of toucans.

Conservation status

Of the 383 piciform species, a total of 15 species are classified as Critically Endangered, Endangered, or Vulnerable. An additional 28 species are classified as Near Threatened. All of the most-threatened species show downward population trends. All three species listed as Critically Endangered are large woodpeckers: the imperial woodpecker (Campephilus imperialis), the Okinawa woodpecker (Sapheopipo noguchii), and the ivory-billed woodpecker (Campephilus principalis). (Classification as Critically Endangered means experts believe these species have no more than an estimated 50% chance of surviving over the next 10 years or three generations.) Indeed, the U.S. Fish and Wildlife Service declared the ivory-bill extinct in 1997 because the last confirmed U.S. sighting occurred almost 50 years earlier. In 1999, however, a credible report of an ivory-bill sighting in a Louisiana swamp raised hopes that this species may yet persist. A 2002 expedition to search for the species was inconclusive; no birds were spotted, but experts believe they heard the ivory-bill's distinctive "double-knock" drumming.

Though at present a comparatively small proportion of species are threatened, experts caution against complacency because, almost everywhere these birds are found, habitat loss and habitat fragmentation are occurring at a rapid rate. When forests are clear-cut for lumber, not only is habitat destroyed in the short term, but subsequent commercial reforestation and lumber management practices produce young, even-aged stands that lack the standing dead trees many species require for nest sites and to provide insect food. Meanwhile, forest fragmentation resulting from agriculture or development activities is a problem for species that require large tracts of unbroken forest. Piciform species that prefer edge habitat, however, may be increasing in numbers; data is lacking.

Habitat loss has already been identified as a factor in the declines of some toucan species—in South America, the saffron toucanet (Baillonius bailloni) is threatened both by hunting and by capture for the cage-bird trade. Forest loss in the tropics is probably a problem for many puffbird species as well, although data is limited. In Indonesia, both logging and fires threaten barbet habitat. Experts believe that the collection of specimens for museums contributed to the extinction of some species.

Significance to humans

The group of birds that scientists have named for a mythical deity are prominently featured in the myths and folklore of many native cultures. Historically, native people have used toucan and woodpecker feathers and beaks as ceremonial ornaments; these large birds were also hunted for food.

With regard to popular culture in the developed world in the twenty-first century, toucans are probably best known as the mascot for a popular breakfast cereal that is (appropriately) fruit-flavored. Woody Woodpecker, the impudent cartoon character with the trademarked laugh, was created in the 1940s by Walter Lantz for Universal Studios and is still popular with children decades later. Meanwhile, their parents may consider woodpeckers to be pests because the birds sometimes damage homes when they drum on roofs or siding—either to signal possession of a territory, or to get at concealed insects infesting the home. Perhaps the most famous case of woodpecker damage occurred in 1995, when northern flickers (Colaptes auratus) in Florida pecked four-inch-diameter holes in the foam insulation covering the fuel tanks of the space shuttle Discovery.

Scientists, however, generally regard the piciform birds as beneficial to ecosystems. For example, fruit-eating species such as toucans and barbets play a key role in maintaining tropical forests because they disperse tree seeds into areas favorable for germination. Woodpeckers help suppress populations of pest insects in forests, and their abandoned nesting cavities provided crucial nest sites for such hole-nesting birds as bluebirds as well as mammals such as flying squirrels. Migratory hummingbirds are often sustained in spring by the insects attracted to the sweet maple sap dripping from holes drilled by sapsuckers.

Resources

Books

Haffer, J. Avian Speciation in Tropical South America. Cambridge, MA: Nuttall Ornithology Club, 1974.

Short, L. Woodpeckers of the World. Delaware Museum of Natural History, 1982.

Sibley, C. G., and B. L. Monroe, Jr. Distribution and Taxonomy of Birds of the World. New Haven: Yale University Press, 1990.

Sibley, C. G., and J. E. Ahlquist. Phylogeny and Classification of Birds: A Study in Molecular Evolution. New Haven: Yale University Press, 1990.

Skutch, A. F. Life of the Woodpecker. Santa Monica: Ibis Publishing Company, 1985.

Periodicals

Hofling, E., and M. F. Alvarenga-Herculano. "A Comparative Study of the Bones of the Shoulder Girdle in the Piciformes, Passeriformes and Caraciiformes, and also in Related Orders of Birds such as the Trogoniformes, Coliiformes, Apodiformes, Strigiformes and Carpimulgiformes." Zoologischer–Anzeiger 240 (2001): 196–208.

Remsen, J. V., Jr., M. A. Hyde, and A. Chapman. "The Diets of Neotropical Trogons, Motmots, Barbets, and Toucans." Condor 95 (1993): 178–192.

Cynthia Ann Berger, MS