It is not only fine feathers that make fine birds. ~ Greek fabulist Aesop
With over 18,000 extant species, birds are the most speciose class of tetrapods. The living descendants of dinosaurs, birds have had ample time to adapt and diversify. Birds live most everywhere that a macroscopic animal might. All are feathered, winged, bipedal, endothermic egg layers, with beaks but no teeth.
Wings are an evolutionary fashion from forelimbs. The now extinct moa of New Zealand, hunted to extermination by humans, was the only known bird without wings. Most birds can fly, but a few, including penguins and endemic island species, forsook flight.
Birds have light but strong skeletons, 4-chambered hearts, and a high metabolic rate. They range in size from the 5 cm, 2 g bee hummingbird to the 2.75 m, 125 kg ostrich.