Birds are bipedal, constant-temperature, egg-laying vertebrates with feathers, forelimbs that evolved into wings and a hard beak. Birds are a subclass of vertebrates in taxonomy. Birds can be traced back to archaeopteryx in Mesozoic and Jurassic. Dictionary explanation: Birds are a kind of vertebrates. They lay eggs with warm blood and breathe with lungs. Almost all of them have feathers. Their hind legs can walk, their forelimbs become wings, and most of them can fly.

Boundary: animals in the animal kingdom

Door: Chordata, Chordata.

A subphylum of vertebrates.

Class: birds among birds

As for why penguins can't fly and get fat, this is the result of their adaptation to the Antarctic environment.

Because of the cold, they have to accumulate thick fat in their bodies to keep out the cold. The weight gain made it difficult for them to fly, and their wings began to degenerate slowly, because they were going to fish in the sea and had thick fins on their feet. This is the result of natural selection!