November092007

Nature Photos of the Week

November 9, 2007

Woodpeckers!

Woodpeckers are near passerine birds of the order Piciformes. They are found worldwide and include about 180 species including the famous Ivory-billed Woodpecker. 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.

Some woodpeckers 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 is not attached to the woodpecker's head in the same way as it is in most birds, but instead it curls back up around its skull, which allows it to be so long.

The woodpecker first locates an insect tunnel 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 series of barbs. With its tongue, the woodpecker hooks 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 are not visible to predators and eggs do not need to be camouflaged. Cavities created by woodpeckers are also reused as nests by other birds, such as chickadees, bluebirds, some ducks and owls, and mammals, such as flying squirrels.

Several adaptations combine to protect the woodpecker's brain from the substantial pounding that the pecking behavior causes: it has a relatively thick skull with relatively spongy bone to cushion the brain; there is 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; and its relatively small brain is less prone to concussion than other animals'. (Schwab, 2002)

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.

The Pileated Woodpecker (Dryocopus pileatus) is our largest woodpecker in the northeast.

Adults (40-49 cm long, 250-350 g weight) are mainly black with a red crest and a white line down the sides of the throat. Adult males have a red line from the bill to the throat and red on the front of the crown. In adult females, these are black. Both sexes show white on the wings in flight. The only North American birds of similar plumage and size are the Ivory-billed Woodpecker of the Southeastern United States and Cuba, and the related Imperial Woodpecker of Mexico. Both of those species are extremely rare, if not extinct.