How woodpeckers repeatedly slam their heads into trees without causing serious brain injury has always fascinated birders and scientists alike. Leading theories suggested that a foamy layer between the birds’ bill and skull helps absorb their impact with wood, protecting their brains. This long-held belief that woodpeckers had built-in shock absorbers has even inspired engineers to develop materials and helmets for injury prevention in contact sports.
However, results from a new study reveal the opposite: The birds actually minimize the need for shock absorption. How? Their heads and beaks essentially act like a stiff hammer, striking and stopping in unison.
“It’s a really sensible advance in our understanding of this really interesting behavior in woodpeckers,” says Tom Roberts, a biologist who studies the biomechanics of drumming in woodpeckers at Brown University.
Woodpeckers use their bills to excavate holes for raising young, forage for insects ensconced in dead limbs, and drum to establish territory. To produce the loudest drums, they ram their bills at rapid speeds—up to 25 times per second—into trees and even utility poles. Excavating nests happens slower, but requires more force: Woodpeckers sometimes slam into trees with 1200g’s of force—greatly surpassing the less than 100g's that can cause concussions in humans.
Given these extreme physical demands, it’s unsurprising that woodpeckers' ability to avoid injury has long sparked curiosity among birders, physicians, biologists, and even engineers. While scientists have had many possible explanations, much of which has been based on modeling, very little real-world data existed to support the theories, says Erica Ortlieb, an author of the new study in Current Biology.
To test the largely assumed theory that woodpeckers absorb shock to protect their brains, researchers used high-speed videos to examine the pecking behavior of three woodpecker species: Black, Pileated, and Great Spotted Woodpeckers. The cameras recorded at up to 4,000 frames per second—much faster than a smartphone’s 30—as they hammered into wood, allowing the researchers to track subtle movements in the birds’ beaks and heads upon impact.
Impressively, a few frames was all it took to capture the milliseconds-long peck. “They stop so suddenly on these pieces of wood,” says Ortlieb, who recorded the Pileated Woodpeckers as a part of her graduate research at University of British Columbia. “It’s incredible.”
Placing small markers on the beak and head of the bird in each still image from the video recording, scientists measured how fast the bill and the head stopped moving after hitting the wood. “There was a lot of manual work looking at these images and actually clicking these spots,” says lead author Sam Van Wassenbergh, a biomechanical biologist at University of Antwerp in Belgium who recorded the Black and Great Spotted Woodpeckers.
If woodpeckers did absorb shock, the researchers would have observed the bill taking the brunt of the impact. But instead, they found that the birds’ beaks and heads stopped at the same rate, indicating that both experienced the same force of impact. “That’s not what we had in mind when we started,” says Robert Shadwick, a co-author and biologist at University of British Columbia. “But I think the ultimate result is totally sensible and it’s obvious.”
To better understand the result, the researchers plugged body measurements from the Black Woodpecker, along with the average speed of the head at impact, into computer models. They tested several models of pecking behavior: one that had no shock absorption, like they found with the video recordings, and several that had a small spring connecting the bird’s bill to the head, mimicking a shock absorber. The team found that the spring severely reduced how hard the bird could peck wood.
“There’s just a very large energetic cost if you want to absorb the shock and still be a good pecker,” Van Wassenbergh says. If the woodpeckers had a built-in cushion to reduce the forces on their brains, they would have to hammer much harder to compensate and still drill holes into wood. Evolutionarily speaking, he says, if absorbing shock made birds worse at drilling for food or excavating nests, it wouldn’t make sense for this function to have evolved.
“The most important part of the study is the insight that it wouldn’t really make sense to absorb all the energy of impact to protect the brain,” Roberts says. “This is real data, for the first time supporting the idea that ‘no you don’t want to absorb all the shock.’”
While this new study squashes the idea that woodpeckers have a built-in shock-absorbing mechanism, the researchers say it just generates more questions about how the birds avoid brain injury. Biologists and engineers, eager to understand the peculiar ability or mimic it for human benefit, have proposed many ideas: cushioning from the jaw or the bizarre, long and curved tongue bone, and engaged neck muscles have all offered alluring explanations.
Future studies could look into the neck muscles and how they flex prior to impact as a contributing factor. “There may be more to that story,” Shadwick says. But, the most compelling possibility, the authors say, is the woodpeckers’ smaller size: Because birds are smaller, they can withstand more force than humans before incurring brain trauma.
Digging into this size idea, the authors looked at how intense the observed forces were—400 to 600g’s in the videos—for the woodpeckers’ brains relative to humans, using more computer modeling. Even though it seems like the birds could be injuring their brains, the results reveal that the woodpeckers perform under a safety threshold; birds would need to either drill twice as fast or hit a much stiffer surface (like metal on a utility pole) to incur any injury. If this theory gains further support, then it could turn out that there is one simple explanation for how woodpeckers avoid injury: physics.