There are eight thousand and six hundred species of birds ranging in size from the hummingbird to the largest ostrich. The birds differ in colors, sounds, environments and food varieties, but they make up a distinct range of animals with many similar characteristics. For example, all birds have feathers and this is enough to determine if the unknown organism is a bird or not. Birds are warm-blooded animals as mammals, and the feathers help them keeping their bodies warm. All birds fly except the ostrich and the penguin.