There are more than 4,000 known species of mammals. From the tiny bumblebee bat to the giant blue whale, all mammals have two characteristics in common that no other animals possess. Mammals have hair and produce milk to nourish their young. Except for the duck-billed platypus and the echidna, mammals bear live young. They are also endothermic, or warm-blooded, as are birds. No animals are more visible than the birds, of which there are more than 9,000 species. The feature unique to birds is the feather. Made of keratin, which is similar to what our fingernails are made of, the feather is lightweight yet strong. Feathers, along with wings, hollow bones, and a highly efficient respiratory system, enable birds to fly. The ability to fly has allowed birds to spread out over the entire globe. .