Birds & Mammals
Birds are vertebrates of the class Aves. There are nearly 10,000 bird species in the world. They are the only vertebrates that have feathers. Birds also have beaks and forelimbs modified into wings. Mammals (Mammalia) are also vertebrates. There are more than 4,000 different species of mammals. But whether they live on land or water, all mammals share some common characteristics. The distinguishing characteristic of mammals is that the mother nourishes newborn young with milk and hair is also common among mammals. All mammals breathe air so even those living in water must surface to take a breath. Like birds, all mammals are warm-blooded (endothermic).