Large birds of prey such as falcons, ospreys, eagles, and vultures can stay in the sky, gliding along rising air currents and flying tens of thousands of kilometers without flapping. Scientists and laymen alike—fascinated by the feat—have wondered for centuries how they did it.
Now an international team of researchers led by evolutionary biologist from the University of Florida, Dr. She tells Emma Schachner that she has finally solved the mystery. She first reported that soaring birds use their lungs to power flight in a way that has evolved over time. The team’s study has just been published in a prestigious journal Nature under the title “The respiratory system affects flight mechanics in soaring birds.”
“Birds are very diverse. Think about how an ostrich is different from a hummingbird or a penguin,” she said. “Their lungs are probably involved in a number of really fascinating functional and behavioral activities that are waiting to be discovered.”
Unlike mammalian lungs, bird lungs do more than just breathe. The air-filled sac in the birds’ lungs is thought to increase the force the birds use to power their flight muscles while gliding.
“Breathing has long been known to be functionally linked to locomotion, and flapping has been shown to increase ventilation,” Schachner said. “However, our findings show that in some species the opposite is also the case. Part of the respiratory system influences and adjusts the performance of the flight apparatus in soaring birds, which use their lungs to modify the biomechanics of the flight muscles.”
Mammalian lungs are flexible and air flows in and out the same way. In contrast, birds have a unique way of breathing – with stationary lungs that pump air through them in one constant direction with a series of balloon-shaped air pockets that expand and deflate. Branching off from these air pockets are many small extensions called diverticula, which vary in number and size in different bird species and whose functions remain poorly understood.
The discovery of a unique air sac known as a subpectoral diverticulum (SPD) happened by chance when Schachner was working on another project involving the anatomy of red-tailed hawks. Buteo jamaicensis and Buteo swainsoni. Looking at the CT scans, she noticed a huge bulge located between the pectoral muscle—the down-swinging muscle—and the supracoracoideus muscle (the up-swinging muscle), both of which are on the front of the bird’s chest. The SPD is an extension of the respiratory system in birds, located between the primary muscles responsible for flapping the wings.
This discovery led Schachner to believe that this air sac could be important to the mechanics of sailing. To test her idea, she collaborated with three key 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 acquired the images for unrelated clinical purposes; and Dr. Karl Bates from the University of Liverpool in the United Kingdom.
Moore and Schachner looked for the presence or absence of air sacs in 68 bird species broadly representative of living avian diversity to assess whether takeoff flight and unique structure are evolutionarily correlated. The dataset consisted mainly of a set of provided micro-CT scans of live birds. Their analyzes were unequivocal: SPD has evolved in hovering lineages at least seven different times and is absent in all non-hovering birds.
Scientists looked at evolutionary patterns
“This evolutionary pattern strongly suggests that this unique structure is functionally important for aerial flight,” Schachner said.
To better understand the air bag’s effect on flight mechanics, Schachner worked with a digital model of its effect on the pectoral muscle, focusing on red-tailed and Swainson’s hawks.
“Measuring SPD behavior in a real hawk as it soars in the sky is almost impossible, so instead we created a computer model of SPD, bones and wing muscles to get a first look at how they might interact.” Bates said. “This computer model also allowed us to change the anatomy of the hawk, specifically remove the SPD – something we can’t do in a real bird – to better understand its impact on flight.”
Computer models suggested that inflating the air bag increased the pectoralis muscle’s leverage much like using a screwdriver to open a can of paint provided better leverage than using a coin.
The team found that the anatomy of the pectoral muscle of soaring birds is very different from that of non-soaring birds in ways that improve force generation. Taken together, these results provide strong evidence that SPD optimizes pectoral muscle function in soaring birds by improving their ability to maintain the wing in a static horizontal position.
“Part of what makes this discovery so important is that it changes the way we think about the interaction between movement and breathing,” Schachner said. “We know from previous studies that locomotion, such as running or wing flapping, improves lung ventilation, but now we have shown the opposite – that the lungs are also able to fundamentally change the way locomotion works in soaring birds.”
Schachner and her team ruled out other possibilities for the SPD function. Looking at CT images of a live, sedated red-tailed hawk as it breathed, they showed that the birds can voluntarily collapse the air sac and still breathe, and can also open and close it independently.
“The evolutionary story here couldn’t be clearer,” Moore said. “Our data suggest that SPD evolves only in soaring birds, at least seven times independently of distantly related soaring lineages.” So whether you’re looking at a western gull, a turkey vulture, a griffon, a bald eagle, or a brown pelican, they all have SPD that improves their ability to soar.” Research also suggests that bird lungs may have many other unknown and interesting non-respiratory functions that we still have to find, Schachner said.