Both mammals and reptiles posses a diaphragm, the muscle separating the thoracic and abdominal cavities. The reptile breathes through contraction of the diaphragmatic muscles, which are attached to the pubis and the liver. This contraction pulls the liver further down into the abdominal cavity, therefore enlarging the thoracic cavity, which expands the lungs. The diaphragm of the reptile must completely separate the thoracic and abdominal cavities for this mechanism to work. In contrast, the bird has no diaphragm and there is no separation of the thoracic and abdominal cavities.
Breathing is accomplished through contraction of the muscles of the rib cage and pelvis.
The pelvic bones of modern perching birds and Archaeopteryx reveal that both probably assisted their breathing while perching by means of muscles attached between their pubis and tail. Through pelvic and tail movements, birds are able to assist their breathing by expanding or contracting their septate lungs. Although the design of the pelvic bones of modern birds and Archaeopteryx are profoundly different, they most likely served the same purpose - to assist breathing during perching. In contrast, the pelvic bones of the theropod dinosaurs look nothing like that of either modern birds or Archaeopteryx , but look very similar to that of modern reptiles, such as the crocodile. There is no way for the pubis of modern reptiles or the theropod dinosaurs to serve as an attachment point for suprapubic muscles to serve in assisting breathing during perching.