Birds (class Aves) are feathered, winged, bipedal,
endothermic (warm-blooded), egg-laying, vertebrate animals. With around 10,000
living species, they are the most speciose class of tetrapod vertebrates. All
present species belong to the subclass Neornithes, and inhabit ecosystems
across the globe, from the Arctic to the Antarctic. Extant birds range in size
from the 5 cm (2 in) Bee Hummingbird to the 2.75 m (9 ft) Ostrich. The fossil
record indicates that birds emerged within theropod dinosaurs during the
Jurassic period, around 160 million years (Ma) ago. Paleontologists regard
birds as the only clade of dinosaurs to have survived the Cretaceous–Paleogene
extinction event 65.5 Ma ago.
Modern birds are characterised by feathers, a beak with no
teeth, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered
heart, and a lightweight but strong skeleton. All living species of birds have
wings—the now extinct flightless moa of New Zealand were the only exception.
Wings are evolved forelimbs, and most bird species can fly. Flightless birds
include ratites, penguins, and a number of diverse endemic island species.
Birds also have unique digestive and respiratory systems that are highly
adapted for flight. Some birds, especially corvids and parrots, are among the
most intelligent animal species; a number of bird species have been observed
manufacturing and using tools, and many social species exhibit cultural
transmission of knowledge across generations.
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