Gainesville, Florida, soaring birds like ospreys, eagles, and vultures can glide effortlessly without flapping. They ride on rising air currents, a marvel that has intrigued scientists for ages. As reported in Nature, led by University of Florida biologist Emma Schachner, an international team has revealed that these birds uniquely use their lungs to enhance their flight.
Unlike mammal lungs, bird lungs have an air sac that helps increase the force powering their flight muscles during soaring. This sac evolved for this purpose.
Emma Schachner, an assistant professor at UF’s College of Veterinary Medicine, explained that while it’s known that breathing supports movement and that flapping enhances airflow, her team found the reverse is true for some birds. They discovered that the respiratory system influences flight muscles in soaring birds.
Unlike mammals, birds have a unique lung structure where air flows in one direction through stationary lungs, propelled by balloon-like air sacs and small extensions called diverticula. While studying red-tailed hawks, the team stumbled upon this discovery, noticing a significant air sac, known as a subpectoral diverticulum, nestled between key flight muscles on the bird’s chest.
Schachner noticed the air sac and theorized it could play a role in soaring. She collaborated with Andrew Moore from Stony Brook University to test this idea. They examined 68 bird species using micro CT scans provided by Scott Echols, D.V.M. They found the air sac, known as the SPD, in soaring birds but not non-soaring ones.
Their analysis indicated that the SPD evolved independently at least seven times in soaring species. This suggests that the structure is crucial for soaring flight, supporting Schachner’s hypothesis.
Schachner collaborated with Karl Bates from the University of Liverpool to understand how the air sac affects flight mechanics. They used computer models to study its impact on the pectoralis muscle in red-tailed and Swainson’s hawks. Bates explained that studying the SPD’s behavior in real hawks is nearly impossible, so they created digital models.
These models allowed them to simulate the effects of the air sac’s inflation, showing it extends the lever arm of the pectoralis muscle. This provides insights into how the SPD enhances flight mechanics, akin to using a screwdriver for better leverage when opening a paint can compared to using a coin.
The research team discovered that the pectoralis muscle in soaring birds differs significantly from that in nonsoaring birds, enhancing force generation. This, combined with the presence of the SPD, suggests that it optimizes the pectoralis muscle’s function by helping birds maintain a steady, horizontal wing position during flight.
Schachner emphasized the significance of this finding in reshaping our understanding of how breathing and movement interact. While previous studies focused on how movement affects breathing, their research shows that lung function can also alter how movement itself operates in soaring birds.
Furthermore, the team confirmed that the SPD’s primary function is linked to flight mechanics. Observing a sedated red-tailed hawk through CT scans, they demonstrated the bird’s ability to control the air sac voluntarily, collapsing or expanding it independently of breathing.
Moore highlighted that the SPD evolves exclusively in soaring birds, independently emerging in at least seven lineages. This adaptation enhances their soaring capabilities across various species, such as Western gulls, turkey vultures, sooty shearwaters, bald eagles, and brown pelicans.
Schachner added that “the research suggests birds’ lungs serve numerous unknown nonrespiratory functions. Given the vast diversity among birds—from ostriches to hummingbirds to penguins—she believes there are likely many fascinating functional and behavioral roles of their lungs yet to be uncovered.”
Journal reference:
- Schachner, E.R., Moore, A.J., Martinez, A. et al. The respiratory system influences flight mechanics in soaring birds. Nature. DOI: 10.1038/s41586-024-07485-y.
