Birds are very interesting creatures who have some similarities to humans. One example of this is they breathe through their lungs. When birds are babies and in their shells, they get oxygen through the pores in the egg shell. Birds get their oxygen by taking it in through the nostrils. One important characteristic of a bird is the two wings, which are used to fly but not all birds can fly. An example of a bird that cannot fly is a penguin or an ostrich. Another characteristics are the beak and the feathers. The beak is used to crack open their egg's shell when they hatch and to catch and eat their food. Some birds have beaks that are made to crack rocks so the birds can get protein from the small pieces of rock.The feathers are strong, lightweight and flexable enough to allow movement. The feathers also hold in the bird's body heat and for some birds, repel water. Birds eat a variety of food depending on their species. Larger birds can eat more protien, when smaller birds eat things like insects, worms, or berries. To reproduce birds lay eggs in nests that they build. These nests shelter the whole family of birds. For the baby to hatch properly the female bird (mom) needs to incubate before the bird is ready to come out of its shell. To survive birds must fly away from their enemies. It is also one of the reasons why they sing. Another reason they sing is to proclaim territory and attract a mate.Birds have a three-part digestive system; the crop (used to store food), the gizzard (used to grind food) and the cloaca (used for excretion and laying eggs). Their hearts have four chambers.They have light, hollow bones which help them fly.They also have an efficient repiratory system which is useful for flying.