Birds are feathered, winged, bipedal, warm-blooded,
egg-laying, vertebrate animals. Around 10,000 living species
makes them the most speciose class of tetrapod vertebrates.
They inhabit ecosystems across the globe, from the Arctic to
the Antarctic. Extant birds range in size from the 2 in Bee
Hummingbird to the 9 ft Ostrich. The fossil record indicates
birds evolved from theropod dinosaurs during the Jurassic
period, around 150–200 million years ago, and the earliest
known bird is the Late Jurassic Archaeopteryx, around
150–145 million years ago. Most paleontologists regard
birds as the only clade of dinosaurs to have survived the
Cretaceous–Tertiary extinction event approximately 65.5
million years 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 was the only exception.
Wings are evolved forelimbs, and most bird species can
fly, with some exceptions, including ratites, penguins, and
a number of diverse endemic island species. Birds also hav
e 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.