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
Handbook of the Birds of the World a non-profit endeavour fuelled by material from more than one hundred contributors from around the world. In early 2013, Lynx Edicions launched the online database HBW Alive, which includes the volume and family introductions and updated species accounts from all 17 published HBW volumes. Since its launch, the taxonomy has been thoroughly revised and updated twice (once for non-passerines and once for passerines), following the publication of the two volumes of the "HBW and BirdLife International Illustrated Checklist of the Birds of the World". The Handbook of the Birds of the World Alive site also provides a free access 'Key