The respiratory system influences flight mechanics in soaring birds

<p>The subpectoral diverticulum (SPD) is an extension of the respiratory system in birds that dives between the primary muscles responsible for flapping the wing. Surveying the pulmonary apparatus in 68 species showed that the SPD was present in virtually all soaring taxa investigated yet absent in non-soarers. We find that this structure independently evolved with soaring flight at least seven times, indicating that the diverticulum may have a functional and adaptive relationship with this flight style. Using the soaring hawks <em>Buteo jamaicensis </em>and <em>B. swainsoni</em> as models, we show that the SPD is not integral for ventilation, that an inflated SPD can increase the moment arm of cranial parts of the pectoralis, and that pectoralis muscle fascicles are significantly shorter in soaring hawks than in non-soaring birds. This coupling of an SPD-mediated increase in pectoralis leverage with force-specialised muscle architecture produces a pneumatic system adapted for the isometric contractile conditions expected in soaring flight. The discovery of a mechanical role for the respiratory system in avian locomotion underscores the functional complexity and heterogeneity of this organ system and suggests that pulmonary diverticula likely have other undiscovered secondary functions. These data provide a mechanistic explanation for the repeated appearance of the SPD in soaring lineages, demonstrating that the respiratory system can be co-opted to provide novel biomechanical solutions to the challenges of flight and thereby influence the evolution of avian volancy.</p>