Woodpeckers boast unique adaptations

Various species of this bird abound, only six are found in Southeast

Posted: Friday, October 16, 2009

Woodpeckers are found around the world, except on some islands. There are said to be about 183 species, ranging in from sparrow-size to raven-size. The woodpecker lineage is old, dating back over 50 million years, and it is distantly related to the neotropical toucans and barbets and the Old World honey guides.

Here in Southeast, we have only six species. Considering the numbers of trees around here, and compared to forests in the Midwest, the abundance of woodpeckers here seems very low indeed.

All woodpeckers nest in cavities they excavate, usually in the trunks or large limbs of trees. They also pound on trees, each species with a characteristic drumming pattern, to signal their presence to others of the same species. Most of them forage chiefly on tree trunks, flaking bark or digging holes in bark and wood in search of insects, using a chisel-shaped bill. But some species have evolved novel variations on this basic theme.

The only relatively common species here is the Red-breasted Sapsucker. Sapsuckers have a very distinctive mode of foraging. They make orderly arrays of shallow pits or wells in the bark of willows, alders, and conifers. Sap oozes into the wells, and these birds then lap it up, along with any stuck insects, with a brushy tongue. They also catch insects on the wing. Some other woodpeckers also eat sap but lack the special brush for lapping it up.

The northern flicker is the largest of our woodpeckers. Most of its foraging is done on the ground (for ants, if there are any), using a long, sticky tongue. It also eats a fair amount of fruit, at least in some areas, more than the other species.

The other four species that are found here all concentrate on insects winkled out of bark and wood (although they also may eat some fruit or sap). The American Three-toed Woodpecker and the Black-backed Woodpecker are the heavy hitters; they forage by drilling strongly into wood, and they also whack off chunks of bark, especially from conifer trees. Hairy woodpeckers are strong excavators too. The small Downy Woodpecker drills into wood but also opens up the stalks of tall weeds to extract insects.

When a woodpecker's bill strikes a tree, it may move at a speed of 20 or more feet a second. With each strike, the force of deceleration has been measured at hundreds of Gs-many times more than the impact force of a peregrine falcon striking its prey. Furthermore, when a woodpecker drills a hole, it may make over 100 strikes a minute. And they often spend many hours a day whacking their bills against wood.

So the question arises: Why don't they get concussions, or at least terrible headaches? We can't say for sure about the headaches, but they do manage to avoid concussion. But how? Why don't we see them staggering around in a daze or flopped on the ground at death's door?

Woodpeckers have several ways to prevent injury when they are drilling a hole in wood. The bill typically strikes at an angle perpendicular to the tree surface, which prevents twisting, and the force of the strike passes through the skull underneath the brain, which thus avoids direct shock. The strongest excavators (such as the Black-Backed and American Three-toed, among our species) have better developed protective arrangements of the skull, compared to sapsuckers and flickers. For these species the front part of the skull and the bony wall between the eyes is reinforced. There are big muscles attached to the base of the bill that act as shock absorbers. A couple of enlarged ribs support very strong neck muscles, which stabilize the neck against the force of the strike.

Woodpecker tongues are long and very extensible. Species that excavate into bark and wood for insects have sharp, barbed tongues for spearing prey in their tunnels. Bird tongues are supported by cartilage and thin, flexible bones. In woodpeckers, these bones extend backward to slide smoothly up over the top of skull when the long tongue is retracted.

The rear portion of the woodpecker body is also modified for their way of life. When a woodpecker clings to a tree trunk, gravity tends to pull the body out, away from the trunk, and down. To counteract this pull, the tail and feet work together. The tail is used as a prop, held against the tree. The tail feathers are stiff, pointed, and mostly black (the melanin pigment helps resist wear). The last several vertebrae in birds are fused together and support the tail, and in excavating woodpeckers that depend on their tail prop, this part of the spine is heftier than in other birds.

Woodpecker feet have several special adaptations to the climbing and hammering habit. When most woodpeckers perch on a branch or hop on the ground, the foot has two toes pointed forward and two toes pointed back (in contrast to songbirds, which typically have three toes forward and one back). As a woodpecker climbs up a tree, two toes point upward, helping resist the pull of gravity. One of the back toes can be rotated out to the side, helping hold the bird's body close to the trunk (and the remaining back toe may not be used at all). But the American Three-toed and Black-backed woodpeckers differ in that one back toe has been lost altogether. The loss of the rear toe somehow allows them to have a stronger stance and a stronger strike on the tree. They rear back and put the weight of the whole body into the strike.

In addition to spreading out the toes to get a better grip on the tree, some woodpeckers also spread out their legs to the side, widening the stance. The bigger the bird, the wider the stance, and the better the hold on the tree.

• Mary F. Willson is a retired professor of ecology.

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