Definition: A bird is an animal that belongs to the Class Aves. Birds are a diverse group of vertebrates that 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 (Gill 1990). 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).
Birds, best known for their ability to fly, are unmatched in their command of the skies. Albatrosses glide long distances over the open sea, hummingbirds hover motionless in mid-air, and eagles swoop down to capture prey with pinpoint accuracy. But not all birds are aerobatic experts. Some species such as kiwis and penguins, lost their ability to fly long ago in favor of lifestyles suited more for land or water.
Birds are the only group of animals to posess feathers. Feathers are used in flight but also provide birds with other benefits such as temperature regulation and coloration (for display and camouflage purposes). Feathers are made of a protein called keratin, a protein that is also found in mammalian hair and reptilian scales.
