This research examines the evolution, phylogenetic distribution, and functional explanations for a peculiar and often overlooked character seen in birds, herein called tracheal and esophageal displacement. Of special interest to this study is examining whether the trait was present in non-avian theropod dinosaurs. This study found that essentially all birds are characterized by a laterally displaced trachea and/or esophagus. The displacement may occur gradually along the neck, or it may happen immediately upon exiting the oropharynx. Displacement of these organs is the result of a heavily modified neck wherein muscles that create mobility restrictions in lizards, alligators, and mammals (e.g., m. episternocleidomastoideus, m. omohyoideus, and m. sternohyoideus) no longer substantially restrict positions in birds. Rather, these muscles are modified, which may assist with making tracheal movements.

An exceptionally well-preserved fossil theropod, Scipionyx samniticus , proved to be paramount. Its in situ tracheal and esophageal positions and detailed preservation (showing the hallmarks of displacement including rotation, obliquity, a strong angle, and a dorsal position in a caudad region of the neck) demonstrate that at least some theropods were characterized by tracheal and esophageal displacement. Ultimately, the presence of the trait correlates with a highly flexible neck, allowing slack and permitting for the organs to save length as they avoid the long curves of the S-shaped neck.