Birds, members of the class Aves, include more than 10,400 living species. Their feathers distinguish them from all other classes of animal; no other animals on earth have them. If you see an animal with feathers, it’s undoubtedly a bird. Like mammals, birds are warm-blooded vertebrates with four- chambered hearts. However, they are more closely related to reptiles and are believed to have evolved from dinosaurs.

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