Bird breathing and singing take advantage of respiratory resonances

The intricacy of birdsong stems from complex neuroinstructions and biomechanics, but the relative contribution of each component is unknown. Fainstein et al. developed a model of bird respiration in an effort to further understand the biomechanics of birdsong.

Birds have rigid lungs and a set of air sacs that compress and expand together. Complete circulation throughout the respiratory system takes two cycles, guaranteeing there is always oxygenated air flowing through the lungs. Aerodynamic valves ensure airflow circulates in just one direction.

The team modeled this process with a mechanical analogy, using a cylinder with three cavities, to mimic the lungs and air sacs, and a piston, to represent the compressing elements (ribcage, sternum, etc.). They found breathing occurred at an aerodynamic resonance of the respiratory system.

“If you pay attention to the natural frequencies of the respiratory system, the nervous system can get a very efficient way of making the air circulate,” said author Gabriel Mindlin.

To inform parameters in their model, the researchers experimentally measured the breathing of canaries. They quantified the air sac pressure and thoracic wall displacement with a pressure transducer and Hall effect sensor, respectively.

During singing, respiratory valves constrict and create oscillations. The scientists found compressing the air sacs leads to high pressures and brings nonlinearities into their model. While they did not conduct experiments during singing, their model predicts the resonances would shift to higher frequencies.

In the future, Mindlin and colleagues aim to investigate whether singing specific syllables takes advantage of the resonance of the respiratory system.

Source: “Birds breathe at an aerodynamic resonance,” by Facundo Fainstein, Sebastián M. Geli, Ana Amador, Franz Goller, and Gabriel B. Mindlin, Chaos (2021). The article can be accessed at https://doi.org/10.1063/5.0069696 .