It's well known that dinosaurs are the ancestors of birds, but mostly due from observing genetic material. Using a closely related group to the dinosaurs; crocodilians, we can observe the physical similarity between birds and crocodiles.
Dinosaurs and Crocodilians are both part of the Archosaur groups. Comparing present day members of each group, we can observe that unidirectional airflow through the lungs was a trait archosaur ancestors obtained. By installing flowmeters into the lungs of crocodiles, researchers were able to observe their airflow as they artificially ventilated their lungs. Another method of observing flow was applying flowmeters, but recording crocodiles naturally breathe. A third method was filling the lungs with fluorescent beads to observe the fluid flow direction in the lungs.
Results from all three methods reveal that crocodiles do have unidirectional flow in their lungs, supporting that unidirectional flow is a common trait in all archosaurs. It also reinforces the idea that avians are descendants of dinosaurs.
Farmer, C. G. "Similarity of crocodilian and avian lungs indicates unidirectional flow is ancestral for Archosaurs." Integrative and comparative biology (2015): icv078.