File:Ostrich egg (left) & elephant bird egg (right).jpg

English: Struthio camelus Linnaeus, 1758 ostrich egg (left - model) & elephant bird egg (right - model) (Field Museum of Natural History, Chicago, Illinois, USA).

The ostrich is the largest living bird species on Earth, although larger ones are known from the historical and paleontological records. A real elephant bird egg weighed about 5.5 times more than an ostrich egg. An adult elephant bird weighed about 4 times that of an adult ostrich.

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Birds are small to large, warm-blooded, egg-laying, feathered, bipedal vertebrates capable of powered flight (although some are secondarily flightless). Many scientists characterize birds as dinosaurs, but this is consequence of the physical structure of evolutionary diagrams. Birds aren’t dinosaurs. They’re birds. The logic & rationale that some use to justify statements such as “birds are dinosaurs” is the same logic & rationale that results in saying “vertebrates are echinoderms”. Well, no one says the latter. No one should say the former, either.

However, birds are evolutionarily derived from theropod dinosaurs. Birds first appeared in the Triassic or Jurassic, depending on which avian paleontologist you ask. They inhabit a wide variety of terrestrial and surface marine environments, and exhibit considerable variation in behaviors and diets.