There are two kinds of newly hatched birds: early hatched birds and late hatched birds. As soon as they hatch, their eyes can be opened, their ears can hear, their legs and feet are strong, their bodies are covered with down, and they peck quickly. The late-hatched bird just hatched, naked and unable to open its eyes. Some of them take days to open, and they have no ability to take care of themselves.

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Birds are feathered oviparous vertebrates, also called birds. Their main feature is that their bodies are streamlined and most of them can fly.

Birds are covered with feathers. Generally, their forelimbs become wings. Their pectoral muscles are developed, their rectum is short, they eat more and digest quickly, that is, their digestive system is developed, which is helpful to lose weight and fly. Their hearts have two atria and two ventricles, and their hearts beat very fast.

The whole weight of the bird falls on the hind limbs, and the bones of the hind limbs are strong. Compared with the bones of the hind limbs of other terrestrial vertebrates, the tarsal bones of birds are stretched and play a role in increasing elasticity.