Soaring birds have a special air sac that helps them fly better

We’ve all seen birds soaring effortlessly in the sky, riding thermal currents without a single flap of their wings. These impressive feats of aerial acrobatics have captivated humans for centuries. But have you ever wondered how these birds manage to stay aloft for so long?

An international team of researchers led by Dr. Emma Schachner from the University of Florida has unveiled a groundbreaking discovery that sheds light on this age-old mystery.

The study reveals that soaring birds possess a unique adaptation within their lungs that enhances their flight capabilities. This adaptation is an air-filled sac, known as the subpectoral diverticulum (SPD), which has evolved over time to optimize the biomechanics of flight in these majestic creatures.

Connection between breathing and flying

“It has long been known that breathing is functionally linked to locomotion, and it has been demonstrated that flapping enhances ventilation,” explained Dr. Schachner.

“But our findings demonstrate that the opposite is also true in some species. We have shown that a component of the respiratory system is influencing and modifying the performance of the flight apparatus insoaring birds, who are using their lungs to modify the biomechanics of their flight muscles.”

Unlike the flexible, tidally ventilated lungs of mammals, birds have a unique respiratory system. Their lungs are stationary, and air is pumped through them in one direction by a series of balloon-like air pockets.

Branching off from these air pockets are small extensions called diverticula, whose functions have remained largely mysterious until now.

Accidental discovery in soaring birds

The discovery of the SPD happened serendipitously while Dr. Schachner was working on an unrelated project involving the anatomy of red-tailed hawks.

As she examined CT scans, she noticed a prominent bulge nestled between the pectoralis (the downstroke flapping muscle) and the supracoracoideus muscle (the upstroke flapping muscle). Intrigued by this unusual structure, Dr. Schachner hypothesized that it might play a role in the mechanics of soaring flight.

To investigate her hypothesis, Dr. Schachner collaborated with Dr. Andrew Moore of Stony Brook University and Dr. Scott Echols, an avian surgery specialist who had amassed a collection of micro CT scans of live birds for clinical purposes.

Together, the experts surveyed the presence or absence of the SPD in 68 bird species representing a broad spectrum of avian diversity.

The role of the subpectoral diverticulum

The results of their analyses were striking: The subpectoral diverticulum had evolved independently in soaring lineages at least seven different times, and it was conspicuously absent in all non-soaring birds.

“This evolutionary pattern strongly suggests that this unique structure is functionally significant for soaring flight,” said Dr. Schachner .

To delve deeper into the SPD’s impact on flight mechanics, Dr. Schachner teamed up with Dr. Karl Bates from the University of Liverpool. They developed a digital model to simulate the SPD’s effect on the pectoralis muscle in red-tailed and Swainson’s hawks.

“Measuring the behavior of the SPD in a real hawk as it soars in the sky is close to impossible, so instead we built a computer model of the SPD, bones, and wing muscles to gain the first insights into how they might interact,” said Dr. Bates.

“This computer model also allowed us to change hawk anatomy, specifically to remove the SPD – again, something we can’t do in a real bird – to further understand its impact on flight.”

Soaring birds’ optimal flight performance

The computer models revealed that inflating the air sac increases the lever arm of the pectoralis muscle, effectively enhancing the force it can generate. This principle is akin to using a screwdriver to open a paint can, where the increased lever arm provides better leverage than using a coin.

Furthermore, the research team found that the anatomy of the pectoralis muscle in soaring birds differs significantly from that of non-soaring birds, with adaptations that improve force generation.

These findings collectively provide compelling evidence that the SPD optimizes the function of the pectoralis muscle in soaring birds, enabling them to maintain a static, horizontal wing position with greater efficiency.

Respiration and locomotion in soaring birds

“Part of what makes this such an important discovery is that it reshapes how we think about the interaction between locomotion and respiration,” noted Dr. Schachner.

“From previous studies, we know that locomotion, like running or wing flapping, enhances lung ventilation. But now we’ve shown the inverse: The lung is also able to fundamentally modify the way that locomotion works in soaring birds.”

The experts meticulously ruled out other possible functions for the SPD. By observing CT scans of a live, sedated red-tailed hawk, they confirmed that the birds can voluntarily collapse the air sac while continuing to breathe, and they can also independently open and close it.

A story of convergent evolution

“The evolutionary story here couldn’t be clearer,” said Dr. Moore. “Our data indicate that the SPD only evolves in birds that soar, and did so at least seven times independently across distantly related soaring lineages.”

“So, whether you’re looking at a Western gull, a turkey vulture, a sooty shearwater, a bald eagle, or a brown pelican, they’ve all got an SPD that improves their ability to soar.”

Untapped potential of bird lungs

This remarkable research not only deepens our understanding of the intricate relationship between respiration and locomotion but also hints at the untapped potential of the avian lung. It suggests that bird lungs may harbor a myriad of other non-respiratory functions yet to be discovered.

“Birds are wildly diverse. Think about how different an ostrich is from a hummingbird or a penguin,” said Dr. Schachner. “It is likely that their lungs are involved in an array of really fascinating functional and behavioral activities that are waiting to be discovered.”

As scientists continue to explore the wonders of avian anatomy and physiology, we can anticipate even more exciting revelations about the hidden talents of these remarkable creatures.

So, the next time you witness a bird effortlessly gliding through the sky, remember that there’s more to their aerial prowess than meets the eye.

Within their lungs lies a remarkable adaptation that has evolved over millennia to optimize their flight performance, a testament to the ingenuity of nature’s design.

The study is published in the journal Nature.

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