Unidirectional airflow, air sacs or the horizontal septum: what does it take to make a bird lung?

In this review, we evaluate the differences between the pulmonary anatomy of birds and other sauropsids, specifically those traits that make the avian respiratory system distinct: a fully decoupled and immobilized, isovolumetric gas-exchanging lung separated from compliant ventilatory air sacs by a horizontal septum. Imaging data, three-dimensional digital anatomical models and dissection images from a red-tailed hawk (Buteo jamaicensis), common ostrich (Struthio camelus), barred owl (Strix varia), African grey parrot (Psittacus erithacus) and zebra finch (Taeniopygia castanotis) are used to demonstrate the anatomical variation seen in the pulmonary air sacs, diverticula and the horizontal septum. We address the current state of knowledge regarding the avian respiratory system and the myriad areas that require further study, including the comparative and quantitative ecomorphology of the bronchial tree and air sacs, the non-ventilatory functions of the sacs and diverticula, the fluid dynamics and anatomical mechanisms underlying unidirectional airflow, post-cranial skeletal pneumaticity, and how all of these factors impact reconstructions of respiratory tissues in extinct archosaurs, particularly ornithodirans (i.e. pterosaurs + non-avian dinosaurs). Specifically, we argue that without evidence for the horizontal septum, a fully avian lung should not be reconstructed in non-avian ornithodirans, despite the presence of post-cranial skeletal pneumaticity.This article is part of the theme issue 'The biology of the avian respiratory system'.