Woodpeckers grunt like tennis players

Woodpeckers can really punch above their weight. Forest area birds can attack a tree for approximately 15 mph with your powerful beaks. To achieve this, woodpeckers essentially become hammers, tensing the muscles of their head, neck, abdomen and tail to keep their body completely rigid as they strike the wood.

Although each blow is driven by the hip flexors and anterior neck muscles, biologists have learned that there is a greater force at work in breathing. Like tennis stars grunt to synchronize and stabilize their core and strike a ball, woodpeckers also synchronize their breathing with the movement when they strike a tree. The results are detailed in a study published today in the journal Journal of Experimental Biology.

“What's interesting about woodpeckers is that they take the pecking behavior we see in all birds and take it to the extreme.” Nicholas Antonsonstudy co-author, NSF postdoctoral fellow at Brown University and integrative biologist, says Popular Science. “These extreme actions are great for study in biology because they give us a better understanding of how normal movements are organized and coordinated during high-intensity tasks.”

Woodpecker's world

Woodpeckers are well adapted to life among trees from beak to toe. Two of them toes point inward and two ends back with sharp claws. With the help of these legs they can climb tree trunks in search of food and shelter.

Most bird species use pecking to find food. However, woodpeckers also use their characteristic pecking style to dig nests and use this drumming to let other animals know what species they are – and that they are ready to defend their territory.

“These drumming interactions are similar to the way other birds sing, and woodpeckers can judge each other's quality based on their drumming skills,” Antonson says. “They also generate very high impact forces when they perform these types of actions, making them particularly difficult to perform.”

Ace grunts

To learn more about how woodpeckers use these muscles to drill into wood, Antonson and his team captured eight wild animals. downy woodpeckers (Picoides pubescens). Over the course of three days, they filmed the birds on high-speed video as they drilled and tapped on a piece of hard wood. During the recordings, the team measured electrical signals in the muscles that control the birds' head, neck, abdomen, tail and leg to see when they contracted when the birds banged their beaks.

They also recorded the air pressure in one section of the six birds' respiratory tracts and the amount of air that two of them exhaled through their voice boxes. This helped them track the birds' breathing before returning to the wild.

The team discovered that their hip flexors and anterior neck muscles are significant. They push the birds forward by digging their beaks into the wood. Other muscles appear to play a more supportive role. Antonson says birds throw their heads back and tense three muscles located at the base of the skull and the back of the neck. They also seem to strain their entire body to transform into a hammer. And breathing helps.

“Woodpeckers exhale with each blow,” Antonson explains. “We show that they exhale when they make contact with the wood, similar to how a tennis player grunts when hitting a ball. This is likely because this athletic strategy stabilizes their core and can increase striking force in both contexts.”

The birds also perfectly synchronized their breathing with each beat, at rates of up to 13 beats per second. They inhaled mini-breaths (about 40 milliseconds) between each quick breath. However, it's a little hard to hear as the grumbling is drowned out by the beating of the drums.

As drummers, woodpeckers are not just “one-hit wonders.” They will fine-tune the strength of their hits depending on whether they hit softly to send a message or hit hard. When the team compared the force of muscle contractions during woodpecker pecking, they found that the front the hip flexor muscle contracts more strongly while the birds were drilling. This reduction helped the birds strike harder. The muscles then relaxed with gentler tapping.

“It's a full-body effort that requires precise coordination of the muscles from the tail to the hips, abs, neck and head in precise coordination with the breath that forges them like nature's hammer and makes these kicks so effective,” Antonson says.

The beat goes on

In future studies, the team hopes to see how this drumming differs among other species of woodpeckers. Some of them can attack even faster than the downy woodpeckers in this study.

“One species is capable of drumming at speeds of up to 40 beats per second,” Antonson says. “It would be interesting to see whether the muscle coordination and per-beat exhalation that we observed in our study is maintained at an even higher level, or whether this species potentially uses a different physiological strategy.”