Birds and crocodiles are part of the spectacularly diverse group of animals called the Archosauria, or archosaurs if you’re on casual terms with them. Other (extinct) archosaurs include the dinosaurs (non-avian), pterosaurs, and sundry wondrous other beasts like aetosaurs and phytosaurs. Archosaurs have, and presumably their common ancestor had, many specialized features of their anatomy that are related to metabolism and locomotion. That’s a big reason why, as a scientist, I love them.
Yet the bird lineage evolved its own extreme specializations, whereas in some (but not all!) ways crocodilians stayed closer to the ancestral state. Here is a great example of one of the major categories of differences between living crocs and birds: the proportions of the respiratory system, from freezer specimens I’ve CT scanned with my former PhD student Vivian Allen, which were part of a paper we published in Anatomical Record back in 2009. We scanned the thawed specimens with and without the lungs inflated (croc results not shown for inflated state). This was easy; we just stuck a syringe into the windpipe and then tied it off once we had pressurized the lungs. [I'm now working with Colleen Farmer and Emma Schachner on using these specimens to learn more about the surprisingly "bird-like" features of croc lungs despite the smaller total volume of the airways; more about that another day... we can do MUCH better than these images!]
Here, the airways are coloured blue/purple and the flesh has been made transparent yellow, while the skeleton is orange. The relatively massive size of the airways is evident in birds, especially the air sacs (side pockets of the lungs/other air passages), whether they are relaxed or inflated. The lungs (purple) aren’t that differently sized in the two animals.
Australian Freshwater crocodile from CT scan:
Junglefowl (“ancestral wild chicken”) from CT scan; relaxed airways:
Junglefowl from CT scan; inflated airways:
(note that the light blue region is the expanded air sacs; the lung in purple hardly changes because it is fairly rigid in birds)