<P> 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). </P> <P> 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 of the family Columbidae, such as pigeons, produce a nutritious crop milk which is fed to their young by regurgitation . </P> <P> The avian stomach is composed of two organs, the proventriculus and the gizzard that work together during digestion . The proventriculus is a rod shaped tube, which is found between the esophagus and the gizzard, that secretes hydrochloric acid and pepsinogen into the digestive tract . The acid converts the inactive pepsinogen into the active proteolytic enzyme, pepsin, which breaks down specific peptide bonds found in proteins, to produce a set of peptides, which are amino acid chains that are shorter than the original dietary protein . The gastric juices (hydrochloric acid and pepsinogen) are mixed with the stomach contents through the muscular contractions of the gizzard . </P> <P> The gizzard is composed of four muscular bands that rotate and crush food by shifting the food from one area to the next within the gizzard . The gizzard of some species of herbivorous birds, like turkey and quails, contains small pieces of grit or stone called gastroliths that are swallowed by the bird to aid in the grinding process, serving the function of teeth . The use of gizzard stones is a similarity found between birds and dinosaurs, which left gastroliths as trace fossils . </P>

What is the top part of a birds stomach called