Bird anatomy Birds have a four-chambered heart, in common with mammals, and some reptiles (mainly the crocodilia). This adaptation allows for an efficient nutrient and oxygen transport throughout the body, providing birds with energy to fly and maintain high levels of activity. A ruby-throated hummingbird's heart beats up to 1200 times per minute (about 20 beats per second). Many birds possess a muscular pouch along the esophagus called a crop. The crop functions to both soften food and regulate its flow through the system by storing it temporarily. The size and shape of the crop is quite variable among the birds. Members
The Origin of Birds feature could not have been lost and regained again over the course of evolution. Based on this law, he therefore rejected the possibility of a direct theropod ancestor of birds, though he acknowledged that theropods and birds must have shared a close relationship. Throughout this section, Heilmann examines several groups of possible ancestors in addition to coelurosaurs, including pterosaurs, predentates, and pseudosuchians. Based on his rejection of theropods due to the wishbone issue as well as what Heilmann saw as striking morphological similarities between the skull of "Archaeopteryx", "Aetosaurus" and "Euparkeria", Heilmann concludes that a pseudosuchian origin of birds is