Birds

Birds are one of the most diverse groups of animals on Earth, with nearly 10,000 known SPECIES spread across every continent, including Antarctica. What makes a bird a bird is a combination of features: they are warm-blooded VERTEBRATES with a four-chambered heart, they have FEATHERS, and they lay hard-shelled eggs. Feathers are unique to birds — no other animal on the planet has them. Birds evolved from THEROPOD dinosaurs roughly 150 million years ago, and the famous fossil ARCHAEOPTERYX is one of the earliest known bird-like creatures ever discovered. Today's birds range from the tiny bee hummingbird, which weighs less than a penny, to the towering ostrich, which can stand nearly 9 feet tall.