Woodpeckers grunt like tennis players

Woodpeckers really know how to punch above their weight. The woodland birds can attack a tree at about 15 miles per hour with their powerful beaks. To achieve this, woodpeckers essentially turn themselves into hammers, by bracing their head, neck, abdomen, and tail muscles to hold their bodies completely rigid when they pound into wood. 

While each impact is driven with their hip flexor and front neck muscles, biologists have learned that there is a more breathy force at play here. Like tennis stars grunting to sync and stabilize their core and whack a ball, woodpeckers also synchronize their breathing with their movement when they strike wood. The findings are detailed in a study published today in the Journal of Experimental Biology.

“What is exciting about woodpeckers is that they take the pecking that we see all birds doing and take it to the extreme,” Nicholas Antonson, a study co-author and an NSF Postdoctoral Fellow Brown University and integrative organismal biologist, tells Popular Science. “These extreme performances are great to study in biology because they give us a better understanding of how ordinary movements are organized and coordinated when put to high intensity tasks.”

A woodpecker’s world

Woodpeckers are well adapted to their life among the trees from their beaks down to their feet. Two of their toes point inward and two point rearward with sharp pointed claws. With these feet, they can scale tree trunks in order to find food and shelter.

Most bird species use pecking to find food. However, woodpeckers also use their signature pecking behaviors to dig into nests and use that 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 how other birds sing and woodpeckers can assess each other’s quality based on their skill as drummers,” says Antonson. “They are also generating very high impact forces when they are performing these behaviors, making them particularly challenging to produce.”

Ace grunters

To learn more about how woodpeckers use these muscles when drilling into a tree, Antonson and the team caught eight wild downy woodpeckers (Picoides pubescens). Over three days, they filmed the birds with high-speed video, recording when they drilled and tapped on a piece of hardwood. While recording, the team measured the electrical signals in the muscles that control the birds’ head, neck, abdomen, tail, and leg to see when they contracted as the birds pounded with their beaks.

Additionally, they recorded the air pressure in one section of the airway of six birds and the amount of air two of them exhaled through their voice boxes. This helped them track the birds’  breathing before returning to the wild.

The team found that their hip flexor and front neck muscles are essential. They propel the birds forward as they drive their beaks into wood. The other muscles appear to play more supportive roles. Antonson says that the birds tip their heads back and brace with three muscles that are at the base of the skull and back of the neck. They also appear to brace their whole bodies to turn themselves into a hammer. And breathing helps.

“The woodpeckers exhale on every strike,” explains Antonson. “We show that they are breathing out just as they make contact with the wood, similar to how a tennis player grunts as they strike the ball. This is likely because this athletic strategy stabilizes their core and can boost striking power in both contexts.”

The birds also perfectly synchronized their breathing with each impact, at rates of up to 13 strikes per second. They inhaled mini-breath (about 40 milliseconds) between each rapid blow. However, it is a bit difficult to hear, as the grunts are drowned out by the drumming.

As drummers, woodpeckers aren’t just “one-hit wonders” either. They will fine-tune the power of their impacts, based on if they are tapping more softly to send a message or drilling hard. When the team compared the strength of the muscle contractions as the woodpeckers pecked, they found that the front hip flexor muscle contracted harder while the birds were drilling. This contraction helped the birds drive a harder impact. The muscles then eased when tapping more softly. 

“It’s a whole-body endeavor that requires tight coordination of muscles from the tail, to the hips, abs, neck and head in precision timing with breathing that forge them as nature’s hammer and make these strikes so effective,” says Antonson.

The beat goes on

In future studies, the team hopes to see how this drumming varies among other woodpecker species. Some can strike even faster than the downy woodpeckers in this study. 

“One species is able to drum at rates up to 40 beats per second,” Antonson says. “It would be exciting to see if the muscle coordination and exhaling on each strike we observed in our study hold at that even greater extreme, or if that species potentially uses a different physiological strategy.”