Animals are grouped according to features they have in common. Mammals, amphibians, birds, reptiles and fish are all vertebrates. Mammals are the most advanced class of animal life. They are warm-blooded, which means their body temperature is constant. They are the only animals to produce milk, and only mammals possess true hair or fur.
Most birds can fly.
They have streamlined, feathered bodies and wings. Their legs and feet are scaly and they have beaks for feeding, but no teeth. Birds are warm-blooded and their young hatch out from eggs. Amphibians are born in water and live the early part of their life there. Later they move to land.