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Woodpeckers are near passerine birds of the order Piciformes. They are found worldwide and include about 180 species.
   Woodpeckers gained their English name because of the habit of some species of tapping and pecking noisily on tree trunks with their beaks. This is both a means of communication to signal possession of territory to their rivals, and a method of locating and accessing insect larvae found under the bark or in long winding tunnels in the tree.

Physiology and behaviour

Some woodpeckers and wrynecks in the order Piciformes have zygodactyl feet, with two toes pointing forward, and two backward. These feet, though adapted for clinging to a vertical surface, can be used for grasping or perching. Several species have only three toes. The woodpecker's long tongue, in many cases as long as the woodpecker itself, can be darted forward to capture insects. The tongue isn't attached to the woodpecker's head in the same way as it's in most birds, but instead it curls back up around its skull, which allows it to be so long.
   The woodpecker first locates a by tapping on the trunk. Once a tunnel is found, the woodpecker chisels out wood until it makes an opening into the tunnel. Then it worms its tongue into the tunnel to try to locate the grub. The tongue of the woodpecker is long and ends in a barb. With its tongue the woodpecker skewers the grub and draws it out of the trunk.
   Woodpeckers also use their beaks to create larger holes for their nests which are 15–45 cm (6–18 inches) below the opening. These nests are lined only with wood chips and hold 2–8 white eggs laid by the females. Because the nests are out of sight, they're not visible to predators and eggs don't need to be camouflaged. Cavities created by woodpeckers are also reused as nests by other birds, such as grackles, starlings, some ducks and owls, and mammals, such as tree squirrels.
   Several adaptations combine to protect the woodpecker's brain from the substantial pounding that the pecking behaviour causes: it has a relatively thick skull with relatively spongy bone to cushion the brain; there's very little cerebrospinal fluid in its small subarachnoid space; the bird contracts mandibular muscles just before impact, thus transmitting the impact past the brain and allowing its whole body to help absorb the shock; its relatively small brain is less prone to concussion than other animals'. (Schwab, 2002)
   Some species have modified joints between bones in the skull and upper jaw, as well as muscles which contract to absorb the shock of the hammering. Strong neck and tail-feather muscles, and a chisel-like bill are other hammering adaptations which are seen in most species. Other species of woodpecker, such as the Flicker, uses its long tongue primarily to grab prey from the ground or from under loose bark. It has few shock-absorbing adaptations, and prefers to feed on the ground or to chip away at rotting wood and bark, habits observed in birds outside of the woodpecker family (7). A "continuum" in skull structures, from little- to highly-specialized for pounding is seen in different genera (groups of related species) of woodpeckers alive today.(8) In his classic "Birds of America," John James Audubon describes the slight gradations in hyoid horn length found in different species of living woodpeckers (9). The slack of tongue is kept under the loose skin behind its neck. The tiny bones divide into essentially two tongues, coming back together before entering the beak.

Systematics

The systematics of woodpeckers is quite convoluted. Based on an assumption of unrealistically low convergence in details of plumage and behavior, 5 subfamilies were distinguished. However, it has turned out that similar plumage patterns and modes of life are not reliable to determine higher phylogenetic relationships in woodpeckers, and thus only 3 subfamilies should be accepted.
   For example, the genera Dryocopus (Eurasia and Americas) and Campephilus (Americas) of large woodpeckers were believed to form a distinct group. However, they're quite unrelated and instead close to a Southeast Asian genus, Mulleripicus and Chrysocolaptes, respectively. In addition, the genus allocation of many species, for example the Rufous Woodpecker, has turned out to be in error, and some taxa with unclear relationships could be placed into the phylogeny (Benz et al., 2006; Moore et al., 2006).

Unassigned fossil forms

  • Genus Palaeonerpes (Ogalalla Early Pliocene of Hitchcock County, USA) - possibly dendropicine
  • Genus Pliopicus (Early Pliocene of Kansas, USA) - possibly dendropicine
  • cf. Colaptes DMNH 1262 (Early Pliocene of Ainsworth, USA) - malarpicine?

Species in taxonomic order

Tribe Dendropicini
  • Genus Melanerpes
  • Genus Sphyrapicus
  • Genus Xiphidiopicus
    • Cuban Woodpecker, Xiphidiopicus percussus (Placement in Dendropicini tentative)
  • Genus Dendropicos
  • Genus Dendrocopos
  • Genus Picoides - this genus is in need of revision (Moore et al., 2006). See the genus article for more.
  • Genus Veniliornis Tribe Malarpicini
  • Genus Campethera
  • Genus Geocolaptes
  • Genus Dinopium
  • Genus Meiglyptes
  • Genus Hemicircus (Placement in Malarpicini tentative)
  • Genus Micropternus (formerly in Celeus) Tribe Picini
  • Genus Picus
  • Genus Mulleripicus
  • Genus Dryocopus
  • Genus Celeus
  • Genus Piculus
  • Genus Colaptes Tribe Megapicini
  • Genus Campephilus
  • Genus Chrysocolaptes
  • Genus Reinwardtipicus
  • Genus Blythipicus
  • Genus Gecinulus (Placement in Megapicini tentative)
  • Genus Sapheopipo (Placement in Megapicini tentative) Further Information

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