The respiratory system influences flight mechanics in soaring birds

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 Buteo jamaicensis and B. swainsoni 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.