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The woodpecker’s beak hits the tree perpendicularly so that there’s minimal rotational acceleration exerted on the brain, which is tightly retained within the skull. If the brain were to rotate, nerves and blood vessels would be injured, which happens in road accidents. In addition, the bird closes its eyes, just before its beak hits the tree, so that they don’t pop out!
As the woodpecker’s beak hits the tree, it’s moving at about 4m/s, reaching this speed over a distance of about 5cm. The proper question to ask is why does the woodpecker accelerate its head like that? The answer is that the woodpecker is a bird. It flies, so needs to be light. But, if it hammers into wood, you’d think its head would have to be heavy – like a hammer. That’s a classic trade-off: lightweight to fly versus heavyweight to hammer. The trade-off is resolved because a light object can deliver the impact energy of a hammer if it hits the target at high speed so that the inertial forces are similar. This begs the question: how does the woodpecker manage to accelerate its head so violently?
Strike and rotate
The woodpecker starts each strike by pulling its body towards the tree with its leg muscles, effectively rotating it about a horizontal axis. The head, on its long neck, follows like a whiplash 50 milliseconds later. In this fashion the woodpecker can use its body muscles to accelerate its head. The tail feathers are very robust and pressed tightly against the tree. They add to the acceleration by bending and storing elastic energy, releasing it as they unbend in the first stages of the stroke.
These combined actions turn the essentially sinusoidal rocking of the woodpecker’s 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. So just as, when delivering a ball, a bowler rotates the body with the arm lagging and then accelerating the action, so the woodpecker is ‘bowling’ its head at the tree. It delivers fracture energy kinetically rather than using inertia like a hand-held hammer.
High-speed blows
This suggests a design for a lightweight hammer. A motor in the handle produces an oscillation that drives a flexibly attached, low-mass hammer head rapidly back and forth, delivering a succession of high-speed blows to its target. Such a hammer can work in confined or difficult spaces. Let me know if you build a prototype!
The woodpecker is solving a trade-off experienced by a lightweight machine. It’s by no means the only example of a trade-off in biology. Evolutionary change in biology results largely from the resolution of thousands of trade-offs over long periods. The best trade-offs survive. Remarkably, in defining a problem, trade-offs are generic, providing bridges between areas such as biology, engineering, physiology, medicine, economics (think Pareto)... speed v accuracy, stiff v ductile, reproduction v immunity, light v heavy, and so on.
This approach can solve technical design problems. Describe your problem in terms of a trade-off, then look for the same trade-off in biology. There’s more on my website. Nearly forgot – the woodpecker holds on to the tree with large, strong, sharp claws.
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Content published by Professional Engineering does not necessarily represent the views of the Institution of Mechanical Engineers.