Woodpeckers and their hammer

David Beeson, with thanks to Julian Vincent’s article in Professional Engineering magazine (Issue 6, 2022). http://www.imeche.org

Tony Cox image.

We have all three British woodpeckers living here. The green woodpecker patrols the lawn seeking out ants, the great spotted woodpecker hammers our trees after woodboring beetle larvae, while the lesser spotted woodpecker attacks the finer decaying branches of birch trees. (The last one is heard rather than seen by myself.)

Great spotted woodpecker: This woodpecker occurs in all types of woodlands and eats a variety of foods, being capable of extracting seeds from pine cones, insect larvae from inside trees or eggs and chicks of other birds from their nests. It can also be spotted eating carrion.

It is a long-lived species with individuals reaching nearly 12 years.

As any bird feeder will know already, fat-rich plant products such as peanuts (and conifer seeds) are particularly important as winter food in the north of the woodpecker’s range, and can then supply more than 30% of the bird’s energy requirements. Other plant items consumed include buds, berries and tree sap, the latter obtained by drilling rings of holes around a tree trunk.

The species breeds in holes excavated in living or dead trees, with the cavity unlined apart from wood chips.

The question is: How does the great spotted woodpecker cope with such a demanding ecological niche? How does the bird manage to hit its head so hard against unforgiving wood yet not injure its brain and still gain access to its food?

Julian’s excellent article explains that the animal’s brain is tightly wrapped within the skull. This ensures no brain movement, plus much less rotation and that avoids the nerve and blood vessel destruction that is so damaging for a human in a car crash. To stop its eyes from popping out on impact, the eyes are momentarily closed as well.

The woodpecker’s beak hits the tree at 9 miles per hour, 4m/sec. Reaching that speed in 2 inches, 5cm. A fast hit in a short distance. (Go on, try running your car into a solid tree at that speed!)

If a human were asked to perform a similar task of boring into wood, they would probably use a heavy pointed hammer. Yet that option is not available to a flying bird … the head would be too heavy to allow it to fly! Birds MUST be light. So, high acceleration and a fast impact is the only option available. And this technique produces a high-energy impact capable of penetrating the bark and wood of the tree.

It seems that the bird first starts its strike by pulling its body against the tree with its powerful leg muscles, causing its flexible, bending, robust tail to store elastic energy. The head is thus held away from the trunk.

With its neck muscles propelling its beak forward, the stored lower body energy is released into greater acceleration and impact energy. Julian says, “These combined actions turn the essentially sinusoidal rocking of the body into an asymmetric sawtooth pattern of the head’s motion, so amplifying the power available. This technique yields 20 times more impact energy than the neck muscles could.” Without this, the bird would never access its food.

With shock-absorbing features and nostrils designed to exclude flying debris the greater spotted woodpecker is quite an animal.

So, next time you encounter a dead woodpecker look at its design: long, sharp claws for adhesion to the tree trunk (two forward and two rear-facing toes), strong but flexible tail feathers and a lightness to its body. (Indeed, looking at a bird’s feet is always instructive – compare water and wetland species’ feet with perching types.)

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