Large birds of prey such as falcons, ospreys, eagles and vultures can remain aloft, glide along rising air currents and fly over tens of thousands of kilometers while barely fluttering. Scientists and laypeople alike – fascinated by this feat – have wondered for centuries how they accomplished it.
Now an international team of researchers led by evolutionary biologist Dr. Emma Schachner from the University of Florida says she has finally solved the mystery. She has reported for the first time that soaring birds use their lungs to increase their flying skills 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 extremely diverse. “Just think how different an ostrich is from a hummingbird or a penguin,” she said. “Their lungs are likely involved in a variety of truly fascinating functional and behavioral activities waiting to be discovered.”
Unlike mammalian lungs, bird lungs do more than just breathe. An air-filled sac in the birds’ lungs is thought to increase the force the birds use to power the flight muscles while flying.
“It has long been known that breathing is functionally linked to locomotion, and flapping has been proven to improve ventilation,” Schachner said. “But our findings show that in some species the opposite is also true. Part of the respiratory system influences and modifies flight apparatus performance in soaring birds, which use their lungs to alter the biomechanics of their flight muscles.”
Mammals’ lungs are flexible and air flows in and out along the same path. Birds, on the other hand, have a unique way of breathing – with a stationary lung through which air is pumped in one constant direction by a series of balloon-like air sacs that expand and deflate. From these air sacs branch off many small extensions called diverticula, which vary in number and size among bird species and whose functions are still poorly understood.
The discovery of the unique air sac known as a subpectoral diverticulum (SPD) occurred accidentally while Schachner was working on another project involving the anatomy of red-tailed hawks. Buteo jamaicensis And Buteo swainsoni. When she looked at CT scans, she saw a huge bulge between the pectoral muscle (the downstroke muscle) and the supracoracoideus (upstroke muscle), both of which are located at the front of the bird’s chest. The SPD is an extension of the respiratory system in birds and is found between the primary muscles responsible for wing flapping.
The discovery led Schachner to suggest that this air sac could be important for the mechanics of flight. To test her idea, she worked with three key collaborators: Dr. Andrew Moore, an evolutionary biologist at Stony Brook University in New York, and veterinarian Dr. Scott Echols, a bird surgery specialist in Utah, who obtained the images for unrelated clinical studies. purposes; and dr. Karl Bates, from the University of Liverpool in Great Britain.
Moore and Schachner looked for the presence or absence of air sacs in 68 bird species that broadly represent the diversity of living birds to assess whether soaring flight and unique structure are evolutionarily correlated. The dataset consisted mainly of a collection of micro-CT scans of live birds. Their analyzes were unequivocal: the SPD has evolved at least seven different times in ascending lineages, and is absent in all non-ascending birds.
Researchers looked at evolutionary patterns
“This evolutionary pattern strongly suggests that this unique structure is functionally significant for gliding flight,” says Schachner.
To better understand the impact of the air sac on the mechanics of flight, Schachner worked to digitally model its effect on the pectoral muscle, focusing on red-tailed hawks and Swainson’s hawks.
“Measuring the behavior of the SPD in a real hawk as it soars through the air is virtually impossible, so instead we built a computer model of the SPD, bones and wing muscles to gain the first insights into how they behave. can interact with each other. Bates said. “This computer model also allowed us to alter the hawk’s anatomy, specifically to remove the SPD – something we cannot do in a real bird – to better understand its impact on flight.”
The computer models suggested that inflating the air bag increases the lever arm of the pectoral muscle, just as using a screwdriver to open a paint can provides better leverage than using a coin.
The team found that the pectoral muscle anatomy 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 the 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 such an important discovery is that it reshapes the way we think about the interaction between locomotion and breathing,” Schachner said. “We know from previous studies that locomotion, like running or flapping the wings, improves lung ventilation, but now we have shown the opposite: that the lung is also able to fundamentally change the way locomotion works in soaring birds. “
Schachner and her team ruled out other possibilities for the SPD position. By looking at CT scans of a live, anesthetized red-tailed hawk as it breathed, they showed that the birds can voluntarily collapse the air sac and still breathe, as well as open and close it on their own.
“The evolutionary story here couldn’t be clearer,” Moore said. “Our data indicate that the SPD only evolves in birds that fly, at least seven times independently, across distantly related flying 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 an SPD that enhances their ability to fly. The research also suggests that bird lungs may have many other unknown and interesting non-respiratory functions that we have yet to find, Schachner said.