Birds are flying, warm blooded vertebrates that have more than 10,400 species that fall under the class Aves. They have two eyes, a four-chambered heart like human beings and forelimbs modified into feathery wings. Birds use their forelimbs for flying purpose and hind limbs for resting or perching. Birds lay hard shelled eggs that need warm conditions to hatch.
Even with such a large number of species, there are approximately 1000 extinct species of birds acquired from fossils.