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).