They are also the first warm-blooded vertebrates so far. In other words, a bird's body temperature stays the same, no matter how cold or hot the air may be. A bird's feathers help to keep the bird warm. The most complex animals are mammals. Mammals are vertebrates with fur or hair.
Female mammals can produce milk for their young and give live birth.
Mammals are also warm-blooded. Mammals are adapted to live in every environment on earth - ice, desert, prairie, jungle, mountain, ocean, fresh water. There are many kinds of mammals. They range from mice to elephants and whales. Bats are mammals that fly.