Woodpeckers

Woodpeckers are part of the bird family Picidae, which also includes the piculets, wrynecks, and sapsuckers. Members of this family are found worldwide, except for Australia, New Guinea, New Zealand, Madagascar, and the extreme polar regions. Most species live in forests or woodland habitats, although a few species are known that live in treeless areas, such as rocky hillsides and deserts, and the Gila woodpecker specialises in exploiting cacti.

Members of this family are chiefly known for their characteristic behaviour. They mostly forage for insect prey on the trunks and branches of trees, and often communicate by drumming with their beaks, producing a reverberatory sound that can be heard at some distance. Some species vary their diet with fruits, birds' eggs, small animals, tree sap, human scraps, and carrion. They usually nest and roost in holes that they excavate in tree trunks, and their abandoned holes are of importance to other cavity-nesting birds. They sometimes come into conflict with humans when they make holes in buildings or feed on fruit crops, but perform a useful service by their removal of insect pests on trees.

The Picidae are one of nine living families in the order Piciformes, the others being barbets (comprising three families), toucans, toucan-barbets, and honeyguides, which (along with woodpeckers) comprise the clade Pici, and the jacamars and puffbirds in the clade Galbuli. DNA sequencing has confirmed the sister relationships of these two groups. The family Picidae includes about 240 species arranged in 35 genera. Almost 20 species are threatened with extinction due to loss of habitat or habitat fragmentation, with one, the Bermuda flicker, being extinct and a further two possibly being so.

General characteristics

Woodpeckers include the tiny piculets, the smallest of which appears to be the bar-breasted piculet at in length and a weight of . Some of the largest woodpeckers can be more than in length. The largest surviving species is the great slaty woodpecker, which weighs on average and up to , and measures , but the extinct imperial woodpecker, at , and ivory-billed woodpecker, around and , were probably both larger.

The plumage of woodpeckers varies from drab to conspicuous. The colours of many species are based on olive and brown and some are pied, suggesting a need for camouflage; others are boldly patterned in black, white, and red, and many have a crest or tufted feathers on their crowns. Woodpeckers tend to be sexually dimorphic, but differences between the sexes are generally small; exceptions to this are Williamson's sapsucker and the orange-backed woodpecker, which differ markedly. The plumage is moulted fully once a year apart from the wrynecks, which have an additional partial moult before breeding.Gorman 2014, pp. 22–23

Woodpeckers, piculets, and wrynecks all possess characteristic zygodactyl feet, consisting of four toes, the first (hallux) and the fourth facing backward and the second and third facing forward. This foot arrangement is good for grasping the limbs and trunks of trees. Members of this family can walk vertically up tree trunks, which is beneficial for activities such as foraging for food or nest excavation. In addition to their strong claws and feet, woodpeckers have short, strong legs. This is typical of birds that regularly forage on trunks. Exceptions are the black-backed woodpecker and the American and Eurasian three-toed woodpeckers, which have only three toes on each foot. The tails of all woodpeckers, except the piculets and wrynecks, are stiffened, and when the bird perches on a vertical surface, the tail and feet work together to support it.

Woodpeckers have strong bills that they use for drilling and drumming on trees, and long, sticky tongues for extracting food (insects and larvae).Winkler, Hans & Christie, David A. (2002), "Family Picidae (Woodpeckers)" ''in'' del Hoyo, J.; Elliot, A. & Sargatal, J. (editors). (2002). '' Handbook of the Birds of the World. Volume 7: Jacamars to Woodpeckers.'' Lynx Edicions. Woodpecker bills are typically longer, sharper, and stronger than the bills of piculets and wrynecks, but their morphology is very similar. The bill's chisel-like tip is kept sharp by the pecking action in birds that regularly use it on wood. The beak consists of three layers; an outer sheath called rhamphotheca, made of scales formed from keratin proteins, an inner layer of bone which has a large cavity and mineralised collagen fibers, and a middle layer made of porous bone which connects the two other layers. Furthermore, the tongue bone (or hyoid bone) of the woodpecker is very long, and winds around the skull through a special cavity, thereby cushioning the brain. Combined, this anatomy helps the beak absorb mechanical stress. Species of woodpecker and flicker that use their bills in soil or for probing as opposed to regular hammering tend to have longer and more decurved bills. Due to their smaller bill size, many piculets and wrynecks forage in decaying wood more often than woodpeckers. Their long, sticky tongues, which possess bristles, aid these birds in grabbing and extracting insects from deep within a hole in a tree. The tongue was reported to be used to spear grubs, but more detailed studies published in 2004 have shown that the tongue instead wraps around the prey before being pulled out.

Many of the foraging, breeding, and signaling behaviors of woodpeckers involve drumming and hammering using their bills. To prevent brain damage from the rapid and repeated powerful impacts, woodpeckers have a number of physical features that protect their brains. These include a relatively small and smooth brain, narrow subdural space, little cerebrospinal fluid surrounding it to prevent it from moving back and forth inside the skull during pecking, the orientation of the brain within the skull (which maximises the contact area between the brain and the skull) and the short duration of contact. The skull consists of strong but compressible, sponge-like bone, which is most concentrated in the forehead and the back of the skull. Another anatomical adaptation of woodpeckers is the enormously elongated hyoid bone which subdivides, passes on either side of the spinal column and wraps around the brain case, before ending in the right nostril cavity. It plays the role of safety-belt.

Computer simulations have shown that 99.7% of the energy generated in pecking is stored in the form of strain energy, which is distributed throughout the bird's body, with only a small remaining fraction of the energy going into the brain. The pecking also causes the woodpecker's skull to heat up, which is part of the reason why they often peck in short bursts with brief breaks in between, giving the head some time to cool. During the millisecond before contact with wood, a thickened nictitating membrane closes, protecting the eye from flying debris. These membranes also prevent the retina from tearing. Their nostrils are also protected; they are often slit-like and have special feathers to cover them. Woodpeckers are capable of repeated pecking on a tree at high decelerations on the order of (1000 g).

Some large woodpeckers such as '' Dryocopus'' have a fast, direct form of flight, but the majority of species have a typical undulating flight pattern consisting of a series of rapid flaps followed by a swooping glide. Many birds in the genus '' Melanerpes'' have distinctive, rowing wing-strokes while the piculets engage in short bursts of rapid direct flight.

Distribution, habitat, and movements

Global distribution

Woodpeckers have a mostly cosmopolitan distribution, although they are absent from Australasia, Madagascar, and Antarctica. They are also absent from some of the world's oceanic islands, although many insular species are found on continental islands. The true woodpeckers, subfamily Picinae, are distributed across the entire range of the family. The Picumninae piculets have a pantropical distribution, with species in Southeast Asia, Africa, and the Neotropics, with the greatest diversity being in South America. The second piculet subfamily, the Nesoctitinae, has a single member, the Antillean piculet, which is restricted to the Caribbean island of Hispaniola. The wrynecks (Jynginae) are found exclusively in the Old World, with the two species occurring in Europe, Asia, and Africa.Gorman 2014, p. 15

Most woodpeckers are sedentary, but a few examples of migratory species are known, such as the rufous-bellied woodpecker, yellow-bellied sapsucker, and Eurasian wryneck, which breeds in Europe and west Asia and migrates to the Sahel in Africa in the winter. More northerly populations of Lewis's woodpecker, northern flicker, Williamson's sapsucker, red-breasted sapsucker, and red-naped sapsucker all move southwards in the fall in North America. Most woodpecker movements can be described as dispersive, such as when young birds seek territories after fledging, or eruptive, to escape harsh weather conditions. Several species are altitudinal migrants, for example the grey-capped pygmy woodpecker, which moves to lowlands from hills during winter. The woodpeckers that do migrate, do so during the day.

Habitat requirements

Overall, woodpeckers are arboreal birds of wooded habitats. They reach their greatest diversity in tropical rainforests, but occur in almost all suitable habitats, including woodlands, savannahs, scrublands, and bamboo forests. Even grasslands and deserts have been colonised by various species. These habitats are more easily occupied where a small number of trees exist, or in the case of desert species like the Gila woodpecker, tall cacti are available for nesting. Some are specialists and are associated with coniferous or deciduous woodlands, or even, like the acorn woodpecker, with individual tree genera ( oaks in this case). Other species are generalists and are able to adapt to forest clearance by exploiting secondary growth, plantations, orchards, and parks. In general, forest-dwelling species need rotting or dead wood on which to forage.Gorman 2014, p. 18

Several species are adapted to spending a portion of their time feeding on the ground, and a very small minority have abandoned trees entirely and nest in holes in the ground. The ground woodpecker is one such species, inhabiting the rocky and grassy hills of South Africa, and the Andean flicker is another.

The Swiss Ornithological Institute has set up a monitoring program to record breeding populations of woodland birds. This has shown that deadwood is an important habitat requirement for the black woodpecker, great spotted woodpecker, middle spotted woodpecker, lesser spotted woodpecker, European green woodpecker, and Eurasian three-toed woodpecker. Populations of all these species increased by varying amounts from 1990 to 2008. During this period, the amount of deadwood in the forest increased and the range of the white-backed woodpecker enlarged as it extended eastwards. With the exception of the green and middle-spotted woodpeckers, the increase in the amount of deadwood is likely to be the major factor explaining the population increase of these species.

Behaviour

Most woodpeckers live solitary lives, but their behaviour ranges from highly antisocial species that are aggressive towards their own kind, to species that live in groups. Solitary species defend such feeding resources as a termite colony or fruit-laden tree, driving away other conspecifics and returning frequently until the resource is exhausted. Aggressive behaviours include bill pointing and jabbing, head shaking, wing flicking, chasing, drumming, and vocalisations. Ritual actions do not usually result in contact, and birds may "freeze" for a while before they resume their dispute. The coloured patches may be flouted, and in some instances, these antagonistic behaviours resemble courtship rituals.

Group-living species tend to be communal group breeders. In addition to these species, a number of species may join mixed-species foraging flocks with other insectivorous birds, although they tend to stay at the edges of these groups. Joining these flocks allows woodpeckers to decrease their anti-predator vigilance and increase their feeding rate. Woodpeckers are diurnal, roosting at night inside holes and crevices. In many species the roost will become the nest-site during the breeding season, but in some species they have separate functions; the grey-and-buff woodpecker makes several shallow holes for roosting which are quite distinct from its nesting site. Most birds roost alone and will oust intruders from their chosen site, but the Magellanic woodpecker and acorn woodpecker are cooperative roosters.Gorman 2014, pp. 19–20

Drumming

Drumming is a form of nonvocal communication used by most species of woodpeckers, and involves the bill being repeatedly struck on a hard surface with great rapidity. After a pause, the drum roll is repeated, with each species having a pattern that is unique in the number of beats in the roll, the length of the roll, the length of the gap between rolls, and the cadence. The drumming is mainly a territorial call, equivalent to the song of a passerine.Gorman 2014, p. 28 Woodpeckers choose a surface that resonates, such as a hollow tree, and may use man-made structures such as gutters and downpipes. Drumming serves for the mutual recognition of conspecifics and plays a part in courtship rituals. Individual birds are thought to be able to distinguish the drumming of their mates and those of their neighbours. Drumming in woodpeckers is controlled by a set of nuclei in the forebrain that closely resemble the brain regions that underlie song learning and production in many songbirds.

Calls

Woodpeckers do not have such a wide range of songs and calls as do passerine birds, and the sounds they make tend to be simpler in structure. Calls produced include brief, high-pitched notes, trills, rattles, twittering, whistling, chattering, nasal churrs, screams, and wails. These calls are used by both sexes in communication and are related to the circumstances of the occasion; these include courtship, territorial disputes, and alarm calls. Each species has its own range of calls, which tend to be in the 1.0 to 2.5 kHz range for efficient transmission through forested environments. Mated couples may exchange muted, low-pitched calls, and nestlings often issue noisy begging calls from inside their nest cavity. The wrynecks have a more musical song, and in some areas, the song of the newly arrived Eurasian wryneck is considered to be the harbinger of spring. The piculets either have a song consisting of a long, descending trill, or a descending series of two to six (sometimes more) individual notes, and this song alerts ornithologists to the presence of the birds, as they are easily overlooked.

Diet and feeding

Most woodpecker species feed on insects and other invertebrates living under bark and in wood, but overall, the family is characterized by its dietary flexibility, with many species being both highly omnivorous and opportunistic. The diet includes ants, termites, beetles and their larvae, caterpillars, spiders, other arthropods, bird eggs, nestlings, small rodents, lizards, fruit, nuts, and sap. Many insects and their grubs are taken from living and dead trees by excavation. The bird may hear sounds from inside the timber indicating where creating a hole would be productive. Crustaceans, molluscs, and carrion may be eaten by some species, including the great spotted woodpecker, and bird feeders are visited for suet and domestic scraps.

Other means are also used to garner prey. Some species, such as the red-naped sapsucker, sally into the air to catch flying insects, and many species probe into crevices and under bark, or glean prey from leaves and twigs. The rufous woodpecker specialises in attacking the nests of arboreal ants, and the buff-spotted woodpecker feeds on and nests in termite mounds. Other species, such as the wrynecks and the Andean flicker, feed wholly or partly on the ground.

Ecologically, woodpeckers help to keep trees healthy by keeping them from suffering mass infestations. The family is noted for its ability to acquire wood-boring grubs from the trunks and branches, whether the timber is alive or dead. Having hammered a hole into the wood, the prey is extracted by use of a long, barbed tongue. Woodpeckers consume beetles that burrow into trees, removing as many as 85% of emerald ash borer larvae from individual ash trees.

The ability to excavate allows woodpeckers to obtain tree sap, an important source of food for some species. Most famously, the sapsuckers (genus ''Sphyrapicus'') feed in this fashion, but the technique is not restricted to these, and others such as the acorn woodpecker and white-headed woodpecker also feed on sap. The technique was once thought to be restricted to the New World, but Old World species, such as the Arabian woodpecker and great spotted woodpecker, also feed in this way.

Breeding

All members of the family Picidae nest in cavities, nearly always in the trunks and branches of trees, well away from the foliage. Where possible, an area of rotten wood surrounded by sound timber is used. Where trees are in short supply, the gilded flicker and ladder-backed woodpecker excavate holes in cactus, and the Andean flicker and ground woodpecker dig holes in earth banks. The campo flicker sometimes chooses termite mounds, the rufous woodpecker prefers to use ants' nests in trees and the bamboo woodpecker specialises in bamboos. Woodpeckers also excavate nest holes in residential and commercial structures and wooden utility poles.

Woodpeckers and piculets excavate their own nests, but wrynecks do not, and need to find pre-existing cavities. A typical nest has a round entrance hole that just fits the bird, leading to an enlarged vertical chamber below. No nesting material is used, apart from some wood chips produced during the excavation; other wood chips are liberally scattered on the ground, thus providing visual evidence of the site of the nest.Gorman 2014, p. 22 Many species of woodpeckers excavate one hole per breeding season, sometimes after multiple attempts. It takes around a month to finish the job and abandoned holes are used by other birds and mammals that are cavity nesters unable to excavate their own holes.

Cavities are in great demand for nesting by other cavity nesters, so woodpeckers face competition for the nesting sites they excavate from the moment the hole becomes usable. This may come from other species of woodpecker, or other cavity-nesting birds such as swallows and starlings. Woodpeckers may aggressively harass potential competitors, and also use other strategies to reduce the chance of being usurped from their nesting sites; for example, the red-crowned woodpecker digs its nest in the underside of a small branch, which reduces the chance that a larger species will take it over and expand it.

Members of Picidae are typically monogamous, with a few species breeding cooperatively and some polygamy reported in a few others. Polyandry, where a female raises two broods with two separate males, has also been reported in the West Indian woodpecker. Another unusual social system is that of the acorn woodpecker, which is a polygynandrous cooperative breeder where groups of up to 12 individuals breed and help to raise the young. Young birds from previous years may stay behind to help raise the group's young, and studies have found reproductive success for the group goes up with group size, but individual success goes down. Birds may be forced to remain in groups due to a lack of habitat to which to disperse.

A pair works together to help build the nest, incubate the eggs, and raise their altricial young. In most species, though, the male does most of the nest excavation and takes the night shift while incubating the eggs. A clutch usually consists of two to five round, white eggs. Since these birds are cavity nesters, their eggs do not need to be camouflaged and the white color helps the parents to see them in dim light. The eggs are incubated for about 11–14 days before they hatch. About 18–30 days are then needed before the chicks are fully fledged and ready to leave the nest. In most species, soon after this, the young are left to fend for themselves, exceptions being the various social species, and the Hispaniolan woodpecker, where adults continue to feed their young for several months. In general, cavity nesting is a successful strategy and a higher proportion of young is reared than is the case with birds that nest in the open. In Africa, several species of honeyguide are brood parasites of woodpeckers.

Systematics and evolutionary history

The Picidae are just one of nine living families in the order Piciformes. Other members of this group, such as the jacamars, puffbirds, barbets, toucans, and honeyguides, have traditionally been thought to be closely related to the woodpecker family ( true woodpeckers, piculets, wrynecks, and sapsuckers). The clade Pici (woodpeckers, barbets, toucans, and honeyguides) is well supported and shares a zygodactyl foot with the Galbuli (puffbirds and jacamars). More recently, several DNA sequence analyses have confirmed that Pici and Galbuli are sister groups.

The name Picidae for the family was introduced by English zoologist William Elford Leach in a guide to the contents of the British Museum published in 1820. The phylogeny has been updated according to new knowledge about convergence patterns and evolutionary history. Most notably, the relationship of the Picinae genera has been largely clarified, and the Antillean piculet was found to be a surviving offshoot of protowoodpeckers. Genetic analysis supports the monophyly of the Picidae, which seem to have originated in the Old World, but the geographic origins of the Picinae is unclear. The Picumninae are returned as paraphyletic. Morphological and behavioural characters, in addition to DNA evidence, highlights genus '' Hemicircus'' as the sister group of all remaining true woodpeckers, besides a sister-group relationship between the true woodpecker tribes Dendropicini and Malarpicini.

The evolutionary history of this group is not well documented, but the known fossils allow some preliminary conclusions; the earliest known modern picids were piculet-like forms of the Late Oligocene, about 25 million years ago (Mya). By that time, however, the group was already present in the Americas and Europe, and they actually may have evolved much earlier, maybe as early as the Early Eocene (50 Mya). The modern subfamilies appear to be rather young by comparison; until the mid-Miocene (10–15 Mya), all picids seem to have been small or mid-sized birds similar to a mixture between a piculet and a wryneck. A feather enclosed in fossil amber from the Dominican Republic, dated to about 25 Mya, however, seems to indicate that the Nesoctitinae were already a distinct lineage by then.

Stepwise adaptations for drilling, tapping, and climbing head first on vertical surfaces have been suggested. The last common ancestor of woodpeckers (Picidae) was incapable of climbing up tree trunks or excavating nest cavities by drilling with its beak. The first adaptations for drilling (including reinforced rhamphotheca, frontal overhang, and processus dorsalis pterygoidei) evolved in the ancestral lineage of piculets and true woodpeckers. Additional adaptations for drilling and tapping (enlarged condylus