What makes a bird, well, a bird? They all share some similar characteristics. Birds are vertebrate animals, meaning they have an internal skeleton and backbone. They share this characteristic with other vertebrate classes such as mammals, fish, reptiles and amphibians. Birds are in the class Aves within the phylum chordata in the animal kingdom. They are warm-blooded, or endothermic, animals that maintain a constant body temperature. Birds have wings, and most of them use those wings to fly. They are covered in feathers, which is probably the most defining bird characteristic. Birds lay eggs, and although the eggs may differ in size, shape, and color, they usually have a similar make-up with a hardened shell to protect the developing baby bird. There are over 10,000 living species of birds that can be found around the world across the continents and in many diverse habitats.