Birds are warm-blooded, bipedal egg-laying vertebrates that are characterized primarily by their physical features of feathers, forelimbs modified as wings, and hollow bones. Birds range largley in size from tiny hummingbirds all the way up to the huge Ostrich. Depending on how you look at it, and what your viewpoints are there are approximately ten thousand different species of living birds. Note the living. there are several others that have become extinct throughout human history.