Definition: A taxonomic group (class) of vertebrates, also known commonly as birds, whose distinguishing characteristics include feathers, endothermy, and the production of amiotic eggs. Birds evolved from reptiles during the Mesazoic Era about 150 million years ago. Today, an estimated 300 billion birds belonging to more than 9000 species inhabit virtually every terrestrial habitat on the planet. There are even many species of birds that are at home in the water as skilled swimmers and divers. Birds range in size from the massive Ostrich (Struthio camelus) to the minute Bee Hummingbird (Mellisuga helenae).
Also Known As: birds