**Birds** are animals very similar to reptiles, but unlike reptiles, birds can maintain their internal body temperature, and they have an outer covering called the *feathers*. They have two legs covered with scales, and two front limbs modified into wings for **flying**.

Birds have the capacity to fly due to their aerodynamic feathers and wings, lightweight bones, strong chest muscles, and highly efficient digestive, respiratory, and circulatory systems. In addition, the energy that they obtain from food and oxygen helps them fly.

**Archaeopteryx** is the first birdlike fossil discovered. It resembled a small running dinosaur however it had well-developed feathers. It contained teeth in its beak, a bony tail, and toes and claws on its wings.

Bird eggs are considered amniotic eggs. Amniotic eggs are characterized by having an albumin and egg yolk which supplies the embryo with necessary fluid, and nutrients. Moreover, amniotic eggs have specialized membranes called the chorion and allantois that facilitates gas exchange and waste disposal.

Most birds incubate and keep their eggs warm until they are ready to hatch. Once the chick is ready to hatch and leave the shell, it will use its small tooth or beak to create a hole in the shell. After so much poking and prodding the shell will then hatch and break releasing the chick.

Since seed-eating birds lack teeth, they have specialized structures which can help digest their food. These structures are the crop, which stores food; and the gizzard, which mechanically breaks down the food particles. Unlike seed-eating birds, the crop and gizzard are not really advantageous to carnivorous birds since they already have their strong hooked bills which can shred the meat into pieces.