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
Human uses of birds whereas an antelope can yield as much as 31,000 kcal/hour, and wild rye around 1,000 kcal/hour. Birds have been domesticated and bred as poultry for use as food for at least four thousand years. The most important species is the chicken. It appears to have been domesticated by 5000 BC in northeastern China, likely for cockfighting, and only later used for food. In ancient Egypt, poultry including ducks, geese, and pigeons were captured in nets and then bred in captivity. Chicken now provides some 20% of the animal protein eaten by the world's human population in the form of meat