Woodpeckers include the tiny piculets, the smallest of which appears to be the bar-breasted piculet at 7.5 cm (3.0 in) in length and a weight of 8.9 g (0.31 oz).[2][3] Some of the largest woodpeckers can be more than 50 cm (20 in) in length. The largest surviving species is the great slaty woodpecker, which weighs 430 g (15 oz) on average and up to 563 g (19.9 oz), and measures 45 to 55 cm (18 to 22 in), but the extinct imperial woodpecker, at 55 to 61 cm (22 to 24 in), and ivory-billed woodpecker, around 48 to 53 cm (19 to 21 in) and 516 g (18.2 oz), were probably both larger.[4][3][5][6][7]
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.[8]
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.[4]
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.[9] Combined, this anatomy helps the beak absorb mechanical stress.[10] 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.[11]
Many of the foraging, breeding, and signaling behaviors of woodpeckers involve drumming and hammering using their bills.[12] To prevent brain damage from the rapid and repeated powerful impacts, woodpeckers have a number of physical features that protect their brains.[13] 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.[13] 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.[9]
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.[14] During the millisecond before contact with wood, a thickened nictitating membrane closes, protecting the eye from flying debris.[15] 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 10,000 m/s2 (33,000 ft/s2) (1000 g).[16]
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.[17]
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.[18] The second piculet subfamily, the Sasiinae, contains the African piculet and two species in the genus Sasia that are found in Southeast Asia.[19] The wrynecks (Jynginae) are found exclusively in the Old World, with the two species occurring in Europe, Asia, and Africa.[18]
Most woodpeckers are sedentary, but a few examples of migratory species are known, such as the rufous-bellied woodpecker, yellow-bellied sapsucker,[18] and Eurasian wryneck, which breeds in Europe and west Asia and migrates to the Sahel in Africa in the winter.[20] 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.[18] 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.[4]
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.[21] 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.[22]
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,[23] and the Andean flicker is another.[22]
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.[24]
Most woodpeckers live solitary lives, but their behavior 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 behaviors include bill pointing and jabbing, head shaking, wing flicking, chasing, drumming, and vocalizations. Ritual actions do not usually result in contact, and birds may "freeze" for a while before they resume their dispute. The colored patches may be flouted, and in some instances, these antagonistic behaviors resemble courtship rituals.[25]
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