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 are large, measuring , and weighing between ; it has been commented in a review that "fork-lift truck book" would be a more appropriate title. As a complement to the "Handbook of the Birds of the World" and with the ultimate goal of disseminating knowledge about the world's avifauna, in 2002 Lynx Edicions started the Internet Bird Collection (IBC). It is a free-access, but not free-licensed, on-line audiovisual library of the world's birds with the aim of posting videos, photos and sound recordings showing a variety of biological aspects (e.g. subspecies, plumages, feeding, breeding, etc.) for every species. It is