New study finds birds of prey use unique air sac to soar to great distances

Large predator birds like falcons, osprey, eagles and vultures — can remain in the sky, gliding along rising air currents and soaring over tens of thousands of kilometers while hardly flapping. Scientists and laymen – fascinated by the feat – have wondered for centuries how they managed to do it. 

Now, an international team of researchers led by University of Florida evolutionary biologist Dr. Emma Schachner, says she has finally solved the mystery. She has reported for the first time that soaring birds use their lungs to boost their flying in a way that has evolved over time. The team’s study has just been published in the prestigious journal Nature under the title, “The respiratory system influences flight mechanics in soaring birds.”

“Birds are wildly diverse. Think about how different an ostrich is from a hummingbird or a penguin,” she said. “Their lungs are probably involved in a variety of really fascinating functional and behavioral activities that are waiting to be discovered.”

Unlike the lungs of mammals, bird lungs do more than just breathe. An air-filled sac within the birds’ lungs is thought to increase the force the birds use to power flight muscles while soaring.

“It has long been known that breathing is functionally linked to locomotion, and it has been proven that flapping enhances ventilation,” Schachner said. “But our findings show that the opposite is also true in some species. A part of the respiratory system influences and modifies the performance of the flight apparatus in soaring birds, that are using their lungs to modify the biomechanics of their flight muscles.”

Mammalian lungs are flexible with air flowing in and out along the same path. In contrast, birds have a unique way of breathing – with a stationary lung that gets air pumped through it in one constant direction by a series of balloon-like air pockets that expand and deflate. Branching off from these air pockets are many small extensions called diverticula that vary by number and size across avian species and whose functions remain poorly understood.

The discovery of the unique air sac known as a subpectoral diverticulum (SPD) occurred by accident as Schachner worked on another project involving the anatomy of red-tailed hawks Buteo jamaicensis and Buteo swainsoni. Looking at CT scans, she noticed a huge bulge that sits in between the pectoralis – the downstroke flapping muscle – and the supracoracoideus muscle (upstroke flapping muscle), both of which are on the front of the bird’s chest. The SPD is an extension of the respiratory system in birds that is found between the primary muscles responsible for flapping the wing.

The discovery led Schachner to suggest that this air sac could be important for the mechanics of soaring. To test her idea, she worked with three main collaborators: Dr. Andrew Moore, an evolutionary biologist at Stony Brook University in New York and veterinarian Dr. Scott Echols, who specializes in avian surgery in Utah, who had obtained the images for unrelated clinical purposes; and Dr. Karl Bates, of the University of Liverpool in the UK.

Moore and Schachner looked for the presence or absence of the air sacs in 68 bird species that broadly represent living avian diversity to assess whether soaring flight and the unique structure are evolutionarily correlated. The dataset predominantly consisted of a collection of micro- CT scans of live birds provided. Their analyses were unequivocal: The SPD has evolved in soaring lineages at least seven different times, and is absent in all non-soaring birds.

Researchers looked at evolutionary patterns 

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

To better understand the air sac’s impact on the mechanics of flight, Schachner worked with to digitally model its effect on the pectoralis muscle, focusing on 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,” Bates said. “This computer model also made it possible for us to change hawk anatomy, specifically to remove the SPD – something we can’t do in a real bird – to better understand its impact on flight.”

The computer models suggested that inflation of the air sac increases the lever arm of the pectoralis muscle much like using a screwdriver to open a paint can provides better leverage than using a coin.

The team found that the anatomy of the pectoralis muscle of soaring birds is very different from that of nonsoaring birds in ways that improve force generation. Taken together, these results provide strong evidence that the SPD optimizes the function of the pectoralis muscle in soaring birds by improving their ability to keep the wing in a static, horizontal position.

“Part of what makes this such an important discovery is that it reshapes how we think about the interaction between locomotion and respiration,” Schachner said. “From previous studies, we know that locomotion, like running or wing flapping, enhances lung ventilation, but now we’ve shown the opposite – that the lung is also able to fundamentally change the way that locomotion works in soaring birds.”

Schachner and her team ruled out other possibilities for the function of the SPD. By looking at CT scans of a live, sedated red-tailed hawk while it breathed, they showed that the birds can voluntarily collapse the air sac and still breathe, and can also independently open and close it. 

“The evolutionary story here couldn’t be clearer,” Moore said. “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.” The research also suggests that birds’ lungs may have many other unknown and interesting nonrespiratory functions that we have yet to find, Schachner said.