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Birds are among the best studied organisms, but there has been much controversy over their origins and classification. With the recent discovery of feathered fossils in China, it is now generally agreed that birds are descended directly from – or actually may be - a group of theropod dinosaurs that evolved during the Mesozoic Era. Birds exhibit many typically reptilian characteristics, including skeletal and blood-cell structures, scales, and embryonic development patterns.
Other avian characteristics common to all birds, including feathers (derived from scales) and bills, are unique. Avian flight probably evolved as a progression from quadrupedal to bipedal locomotion, climbing, leaping, parachuting, and gliding to active flight. Some birds subsequently lost the power of flight as they grew in size or colonized predator-free environments.
Some authorities consider birds to be within the reptilian subclass Diapsida (along with tuataras, lizards and snakes, and crocodilians); others elevate them to their own class, Aves. Approximately 9,700 extant bird species are recognized worldwide; as with other taxonomic groups, this represents a small fraction of the number of species that have existed over time. There are 23 orders falling into two infraclasses. They can be found nearly everywhere on Earth and range in size from the Bee Hummingbird of Cuba (2 g) to the Ostrich of Africa (160 kg). Even the Ostrich is dwarfed by the long-extinct dromornithids of Australia (perhaps 500 kg).
Birds inhabit nearly every conceivable habitat on the earth’s surface and, because of their mobility, have colonized even the most-remote islands. Birds have been recorded flying up to an altitude of over 11,000 m and diving to a depth of 540 m.
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