Birds belong to the vertebrates group. Their principal characteristic is that they can fly. To do this they have an aerodynamic body, and their front limbs have evolved into two wings of identical shape and size. The ostrich, emu and penguin are amongst the few exceptional bird species that cannot fly.
Other characteristics of birds include having a body that is covered in feathers, breathing through lungs, and having a beak instead of teeth. Birds are oviparous, that is to say that after mating they lay eggs and leave them to incubate until their young are born.
Birds are the most abundant species on earth. There are more than 9,000 different types of birds.