Large predatory birds like hawks, ospreys, eagles and vultures can stay in the sky, glide along rising air currents and glide for tens of thousands of miles without flapping their wings. Scientists and laypeople alike – fascinated by this feat – have wondered for centuries how they managed to achieve it.
Now, an international team of researchers led by Dr. Emma Schachner, an evolutionary biologist from the University of Florida, claims to have finally solved the mystery. She reported for the first time that soaring birds use their lungs to accelerate their flight, 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 gliding birds”.
“Birds are extremely diverse. Think about the difference between an ostrich, a hummingbird or a penguin,” she said. “Their lungs are likely involved in a variety of really fascinating functional and behavioral activities that are waiting to be discovered.”
Unlike mammal lungs, bird lungs do more than just breathe. An air-filled sac in birds’ lungs is thought to increase the force that birds use to propel their flight muscles when gliding.
“It has long been known that breathing is functionally linked to locomotion, and flapping has been shown to improve ventilation,” Schachner said. “But our results show that the opposite is also true in some species. Part of the respiratory system influences and modifies the performance of the flight apparatus in gliding birds, which use their lungs to modify the biomechanics of their flight muscles.
Mammalian lungs are flexible and air flows in and out along the same path. In contrast, birds have a unique way of breathing: with a stationary lung in which air is pumped in a constant direction by a series of balloon-shaped air pockets that expand and deflate. Numerous small extensions called diverticula extend from these air pockets, the number and size of which vary among avian species and whose functions remain poorly understood.
The discovery of the unique air sac known as the subpectoral diverticulum (SPD) happened by accident while Schachner was working on another project involving the anatomy of red-tailed hawks. Jamaican Buteo And Buteo Swainsoni. Looking at the CT scans, she noticed a huge bulge located between the pectoralis – the down-beating muscle – and the supracoracoidus muscle (up-beating muscle), both located on the front of the bird’s chest . The SPD is an extension of the respiratory system in birds that lies between the main muscles responsible for flapping the wings.
This discovery led Schachner to suggest that this air sac might be important to the mechanics of gliding. 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 reasons. 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 that largely represent living avian diversity to assess whether soaring flight and unique structure are correlated during evolution. The dataset consisted primarily of a collection of provided micro-CT scans of live birds. Their analyzes were unequivocal: SPD has evolved in gliding lineages at least seven different times and is absent in all non-gliding birds.
Researchers studied evolutionary patterns
“This evolutionary pattern strongly suggests that this unique structure is functionally important for soaring flight,” Schachner said.
To better understand the impact of the air sac on flight mechanics, Schachner worked with it to numerically model its effect on the pectoral muscle, focusing on red-tailed and Swainson’s hawks.
“Measuring how the SPD behaves in a real hawk as it soars through the sky is almost impossible. So we built a computer model of the SPD, wing bones and muscles to get a first look at how they could interact,” Bates said. “This computer model also allowed us to modify the falcon’s anatomy, including removing the SPD – something we can’t do in a real bird – to better understand its impact on flight.”
Computer models suggest that inflating the air sac increases the leverage arm of the pectoral muscle, much like using a screwdriver to open a can of paint provides better leverage than using a coin .
The team discovered that the anatomy of the pectoral muscle of gliding birds is very different from that of non-gliding 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 reshapes the way we think about the interaction between locomotion and respiration,” Schachner said. “We know from previous studies that locomotion, such as running or flapping its wings, improves pulmonary ventilation, but we have now shown the opposite: that the lung is also capable of fundamentally changing the way ventilation locomotion works in soaring birds.”
Schachner and his team ruled out other possibilities for the operation of the SPD. By examining CT scans of a live, sedated red-tailed hawk while it was breathing, they showed that the birds can voluntarily collapse the air sac while breathing, and can also open and close it independently.
“The evolutionary story here couldn’t be clearer,” Moore said. “Our data indicate that SPD evolves only in gliding birds and has done so at least seven times independently across distant gliding lineages. So whether you’re looking at a western gull, a turkey vulture, a sooty shearwater, a bald eagle or a brown pelican, they all have SPD that enhances their ability to glide. Research also suggests that bird lungs may have many other unknown and interesting non-respiratory functions that we have yet to discover. , Schachner said.