Woodpeckers are members of the family Picidae whose distribution stretches from the tundra to the tropics on all continents except Antarctica and Australia. The family Picidae is related to the other families of the Order Piciformes which include jacamars, puffbirds, honeyguides, barbets, and toucans. These relatives can possess traits associated with woodpeckers such as modified foot tendons, features of the palate and sternum, 14 cervical vertebrae, and the absence of downy feathers on adults. Some barbets have thicker bills which allow them to peck wood and excavate their own holes in trees, the only birds other than woodpeckers to do so. Both the honeyguides and woodpeckers possess a very thick skin which protects them from insect bites (such as bites from ants which compose the primary food item in the diet of many woodpeckers). Toucans also nest in tree holes. DNA evidence supports the close relationship between Family Picidae and Family Indicatoridae (honeyguides), both classified in the Infraorder Picides (Winkler 1995; Short, 1982; Harrisson, 1993; Benz, 2006; Webb, 2005; Webb, 2002).
There are about 215 species in the family Picidae. Although they are most commonly associated with hunting insects in wood, they can feed on a variety of food items and some live in grasslands which lack trees. Their modified feet allow for vertical climbing and many use their stiffened tail feathers as props. Woodpeckers nest in holes, usually which they make themselves, where their naked and blind chicks are hatched after a short incubation. The smallest piculets weigh 7 grams and possess winglengths of 44mm while the largest woodpeckers (Campephilus imperialus) are about 2 feet long, weigh about 700 g, and possess wings which measure 313 mm (Winkler 1995; Short, 1982). The earliest fossil woodpeckers are known from the Miocene in Germany, about 25 million years ago (Backhouse, 2005). A slightly older German fossil from the Oligocene (30-34 million years old) is of a small barbet-like bird and is the oldest known woodpecker relative (Mayr, 2005).
Their bill is hard, typically straight, and chisel-tipped. Species which perform more pecking possess a special hinge where the skull meets the bill. This structure, in addition to neck muscles and muscles around the hinge help to redistribute the forces from pecking away from the cranium. The bill not only allows them to make holes in wood to search for insects, but also to excavate a nest and to make the loud “drumming” sounds which only woodpeckers make (Winkler 1995; Short, 1982). Usually a dead branch is used and woodpeckers may strike the wood 20 times per second (Brooke, 1991). Drumming can be used to attract a mate and to establish a territory. In most species, both males and females drum and in some, such as the downy and hairy woodpeckers, there is no apparent difference between the drumming of males and females. In the Lewis woodpecker, only the males drum (Backhouse, 2005).
The ancestral condition of the foot in the order Piciforms is the zygodactyl foot which is adapted for perching rather than climbing. The first toe has been lost several times within the family Picidae (once in some piculets and three separate times in true woodpeckers) (Winkler 1995; Short, 1982).
All members of the family, including wrynecks and piculets, possess an elongated tongue, an elongated hyoid apparatus, and large salivary glands. There has been no serious disagreement with the classification of wrynecks, piculets, and woodpeckers into the same family (Webb, 2005).
The most primitive members of the Family Picidae are the 2 species of wrynecks (Subfamily Jynginae), which are limited to the Old World in their distribution. They possess a four-toed perching foot and move in a way similar to passerine birds. They do not have a pointed bill and do not make their own nest holes, but rather find natural holes in which to nest. Their bill is short. They use their long, sticky tongue to feed on ants. As in most true woodpeckers, both parents care for the eggs/young. Unlike most woodpeckers (whose woodpecking allows them to take advantage of a variety of food sources throughout the year), wrynecks are migratory (Winkler 1995; Short, 1982; Harrisson, 1993).
The geniothyroid is a throat muscle specific to woodpeckers, although it may be homologous to the genioglossus of other birds. Wrynecks have a large muscle and so, if it appears to have evolved a large size early in the woodpecker lineage. Woodpeckers possess an enlarged maxillary gland which is located under the orbit. In wrynecks, the structure of the gland is intermediate compared to higher woodpeckers. An enlarged mandibular gland evolved early in the history of the woodpeckers and is present in all three subfamilies. Wrynecks possess a number of muscular and glandular features which are more like the condition of other bird groups than those of the higher woodpeckers (Goodge, 1972).
The ancestors of wrynecks had already evolved a derived hyoid apparatus and larger glands which apparently originally functioned in ground feeding. Anatomical modifications for life in trees and for pecking evolved afterwards (Goodge, 1972).
DNA evidence supports the position of the wrynecks in the family Picidae as the sister group to both woodpeckers and piculets (Benz, 2006; Webb, 2005).
There are about 30 species of piculets (Subfamily Picumninae), most of which are limited to South America in their distribution. (One species lives in Africa and three in Asia.) They do perform drumming, they do make their own holes, and those that often peck wood possess a bill better suited to chiseling. Unlike wrynecks, piculets have a lifestyle similar to that of woodpeckers. They possess a long tongue with bristles on the tip. While most piculets have 12 tail feathers, two species possess 10 and one possesses 8. Some species, such as Picumnus cirratus can hybridize with other species and can be divided into several races. (Winkler 1995; Short, 1982).
After the divergence of the wryneck lineage, ancestral woodpeckers enlarged the protactor pterygoideus. Many of the muscular and glandular traits of piculets are primitive compared to those of the true woodpeckers. The hyoid horns are short in some piculets (Picumnus cirrhatus, Sasia, and Nesoctites) compared to true woodpeckers and other piculets (Goodge, 1972).
DNA comparisons support the classification of piculets as the sister group of true woodpeckers within the family Picidae (Webb, 2005; DeFillipis, 2000). The subfamily of piculets is divided into two tribes, one of which contains only one species, the Antillean piculet. It is larger than most piculets (up to 28 g). It usually gleans insects off of the surface of bark, only rarely tapping, and includes fruit in its diet. Its lifestyle has been compared to that of barbets and it has been suggested that it is similar to the ancestral form of the woodpeckers (Short, 1982). DNA evidence indicates that this species more closely related to woodpeckers than other piculets (Benz, 2006; Goodge, 1972).
The true woodpeckers are classified in the Subfamily Picinae and their anatomical and genetic relationships indicate that their taxonomy represents a nested hierarchy of descendants from common ancestors. The subfamily can be further divided into 6 tribes and 24 genera and about 180 species (Sibley, 2001; Backhouse, 2005; DeFillipis, 2000; Webb, 2005; Benz, 2006). Hybridization is known to occur between 20 species pairs in 8 genera of woodpeckers, sometimes forming a “superspecies group” (Short, 1982). Genetic evidence indicates that the largest woodpecker genus, Picoides, consists of several groups which are more closely related to other woodpecker genera than they are to each other (Weibel, 2002).
The frontal bone extends over the bill in true woodpeckers and this extension is greater in species that perform the most excavation of wood. Black backed and American three toed woodpeckers have the best skull protection (Backhouse, 2005).
True woodpeckers possess long tongues which have barbs at the tips. Sublingual salivary glands coat the tongue with sticky saliva and species which feed on ants produce a more sticky saliva. The tongue length varies in woodpeckers depending on their diet, as does the hyoid bone to which supports the tongue. The tongue and the hyoid apparatus which supports it may be so long (up to five inches) that they can encircle the skull and end near the eye. (Hummingbirds also possess long tongues and hyoid apparatuses.) Species which hammer at wood more to extract insect larvae tend to have shorter tongues. Some have short tongues, such as downy woodpeckers, while others have longer tongues, such as flickers (Winkler 1995; Short, 1982; Sibley, 2001). Although the woodpecker tongue is clearly highly elongated, it retains essentially the same anatomical features typically found in birds. The muscles which move the tongue still originate on the lower jaw and attach onto these long hyoid horns. At birth, a flicker’s hyoid horns only extend to the back of its skull (Ryan, 2003). The hyoid horns of the yellow-bellied sapsucker extend only to the back of the skull and other woodpeckers possess hyoid horns of an intermediate length (Backhouse, 2005). Long hyoid horns which wrap around the right eye socket have evolved separately at least three times in the family to produce the conditions in Dendrocopos villosus, Hemicircus, and Picumnus. Sapsuckers have short hyoid horns although this may be a derived condition rather than a primitive one (Goodge, 1972).
Woodpecker legs are short and their toes are long and curved. Wrynecks, piculets, flickers, and some of the families of birds related to woodpeckers possess a zygodactyl foot in which two toes are angled backwards (the first and fourth). All woodpeckers utilize this zygodactyl arrangement when they walk on the ground. Most of the true woodpeckers have modified this ancestral arrangement to better accommodate vertical climbing. In this derived structure, called the ectropodactyl foot, the fourth toe is longer and is angled out laterally rather than backward. Eight species of woodpecker modified arrangement further through the loss of the first toe. The largest woodpeckers (of the genus Campephilus) have modified the ectropodactyl foot further so that the first toe is longer with a long claw and is angled upwards so that it lies next to the fourth toe. In these large woodpeckers the long tarsal bones are held against the tree and are covered in a protective callous (Backhouse, 2005).
True woodpeckers evolved stiff tail feathers for support as they climb. To support their vertical feeding habits, woodpeckers possess wider ribs and stronger muscles to support the neck. The pygostyle (the bone composed of fused vertebrae in the tail) of woodpeckers is enlarged to a greater degree than in any other type of bird (although ground-dwelling species like flickers may possess smaller pygostyles). Woodpecker tail muscles are also enlarged (Backhouse, 2005).
While most woodpecker feet are adapted for vertical climbing, this is not true of many ground-dwelling species (such as campo flickers, Andean flickers, and ground woodpeckers). Three lineages of woodpeckers have separately the first toe from their foot. In ground species, the position of the foramen magnum relative to the vertebral column has changed from the right angle typical in climbing species to a more horizontal position.
More than a dozen species of woodpecker often feed on the ground and three species (including the Campo Flicker in the following photo) can inhabit areas which are devoid of trees. Some species make holes in the ground. Terrestrial lifestyles have evolved separately in different groups of woodpeckers. Campo flickers do almost all of their feeding on the ground where they prey on ants and termites (although ovenbird nestlings may also be taken) Salivary glands can be enlarged even more than usual in terrestrial species. (Winkler 1995; Short, 1982; Short 1971).
Woodpecker bills are covered with a hard substance whose cells grow to allow the bill to be self-sharpening. Feathers cover the nostrils to prevent wood chips from entering.
The ability to make holes in wood certainly offered these birds a selective advantage. The ability to find insects in tree wood is an advantage, especially in winter when other food sources are scarce. Many woodpeckers do not migrate but can sustain themselves in their summer habitats. The ability to create a new nest hole not only allows nesting in a more sanitary environment than a nest which has been previously used, predators and other species competing for nest sites are less likely to displace woodpeckers from a new hole. Between 75% and 100% of woodpecker nestings produce at least one fledgling which is much higher percentage than in birds which make open nests (Backhouse, 2005).
Woodpecker nests are often desired by other species and some birds, especially starlings, are known to displace hole-nesters.
Woodpeckers can feed on a variety of items. All species are capable of preying on insects from the ground and the surface of trees and all woodpeckers can eat berries or fruits. All North American species include plant material in their diet. A number of species can even feed on bird eggs and nestlings; the great spotted woodpecker does this the most. In addition to ants, other insects, and spiders, woodpeckers diets have included scorpions, lizards, and mussels. Downy and hairy woodpeckers have been observed to remove fat from carrion and red-bellied and red-headed woodpeckers have scavenged dead fish from shorelines. Red-cockaded woodpeckers are known to ingest raptor feces for the calcium contained in bone fragments.
The majority of woodpeckers can also include plant material in their diet such as fruits, berries, acorns, sap, and nectar. White-headed woodpeckers feed primarily on fruits, seeds, and honey from bees nests which they open. The insect material in their diet includes insects they catch in flight. Acorns may compose half the diet of the populations of the acorn woodpecker which live in the United States while Latinamerican populations include more sap and insects. Ants compose the majority of the diet of many African woodpeckers which often spend much of their time on the ground. For example, 95% of the ground woodpecker’s diet is composed of ants from 8 genera (Backhouse, 2005; Winkler 1995; Short, 1982).Lewis’s woodpecker preys on flying insects moreso than any other woodpecker and its flight pattern is unlike those of other woodpeckers, utilizing increased gliding and maneuverability (Tobaleske, 1996).
Woodpeckers can store nuts and fruit stones in wood crevices and holes that they drill. Acorn woodpeckers may store about 400 acorns per year and one group of acorn woodpeckers stored 220 kg of acorns in an Arizona water tank (Brooke, 1991). Many woodpeckers remove the bark from trees in order to prey on insects underneath. Many woodpeckers use crevices and forks in wood as “anvils” where they can secure food items that they peck to open such as nuts, fruit stones, beetles, and insect galls. The great spotted woodpecker actually make their own anvils out of pine cones and a tree where a great spotted woodpecker feeds may accumulate as many as five thousand of these cones (Winkler 1995; Short, 1982). Red-headed woodpeckers have been observed putting nuts on highways, only to consume the nuts after cars have run over them (Backhouse, 2005).
Males defend territories from rival males and females defend territories from rival females. Some species nest in colonies (for example, Melanerpes striatus groups may form 20 nesting holes in a single tree). Females may initiate courtship by following a male after she enters his territory. In some species, the males feed the females during courtship. All woodpeckers nest in cavities that are rarely reused. The males typically make the nest which typically takes two weeks of work although longer periods and periods of less than a week are possible. The height of woodpecker nests range from 5 to 100 feet above ground (Winkler 1995; Short, 1982). The bills of woodpeckers which perform less excavating is often more curved. A few types of birds other than woodpeckers and piculets are also known to excavate wood (including nuthatches and chickadees) although they can only do so in soft, decomposing wood. (Backhouse, 2005).
Egg number is typically 4-6 although tropical woodpeckers often have fewer eggs per season than those in temperate zones. Some females are indeterminate in their egg laying and will continue to lay eggs if the eggs are removed from the nest. (A female Colaptes auratus once laid 71 eggs in 73 days in this manner; a female Northern flicker laid 70 eggs). Typically, the male cares for the eggs/nestlings at night and both parents care for them during the day. Both males and females incubate the eggs in all species of North American woodpeckers. There are some exceptions; female acorn woodpeckers perform all of the egg incubation and feeding of nestlings and can assist in the construction of the nest. In some species, only female clean the nest. The nestling period typically occurs in 18-35 days. In many species, the young leave the nest soon after their flight feathers form while in others (such as piculets and tropical woodpeckers), the young may remain in the nest until the next breeding season (Winkler 1995; Short, 1982).
The pigments which determine woodpecker coloration are shared between genera and are synthesized through common physiological processes (Stradi, 1998).
Many woodpeckers are affected by habitat destruction. The Japanese pygmy woodpecker, for example, nests primarily in forests of at least 100 hectares (Brooke, 1991).
Woodpeckers such as the red-bellied woodpecker and Northern flicker are among the 200 bird species which perform a behavior known as “anting”. Birds either lay on an ant nest or deliberately place ants in their feathers. The function of this behavior is not known although it may allow ants to remove fungi or parasites (Backhouse, 2005).
Red-headed woodpeckers can have 2 broods per year. Their diet is primarily composed of insects and nuts but it can even include bark, nuts. worms, eggs, bird nestlings, mice, and corn in their diet. They feed on acorns and beechnuts when they are available and can even store acorns and insects. Its diet is one of the most varied among woodpeckers. One third of its diet is composed of animal material; two thirds of it is composed of plant material. They can catch some insects in flight. Red headed woodpeckers migrate long distances, unlike most woodpeckers.
The red-headed woodpecker is not endangered but its declining populations have resulted in its being considered vulnerable (Sibley, 2001). The red-cockaded woodpecker once had a range which covered most of the Southern U.S. and its population was estimated to have been about 1.5 million individuals in the year 1800. The extensive logging of its habitat led to its current fragmented range and the reduction of its population to about 10,000 individuals, a decline of more than 99% (Conner, 2001).
The Ivory billed woodpecker is the largest woodpecker in the U.S. with a length of up to 21 inches long. The Ivory-billed woodpecker was once found in 14 states along the Mississippi and along the Caribbean and South Atlantic coasts. Following the Civil War, deforestation throughout the south from the late 1800s through the early 1900s destroyed much of the mature swamp forest which composed its habitat. By 1937, its range was limited to a small area in Northeast Louisiana. The last sighting in the United States was in the 1950s and the last sighting in Cuba occurred in 1987. In 1999 and the early years of the 21 st century, a few sightings of the woodpecker in Louisiana led the official announcement in 2005 after video footage confirmed the sightings (Hoose, 2004; Backhouse, 2005; Sibley, 2001).
The Imperial Woodpecker was the largest woodpecker in the world. Habitat loss in its native habitat of Western Mexico led to its decline. The last official sighting occurred in 1956 and the last unofficial sighting in 1996 (Backhouse, 2005).