Chickens and other birds are technically reptiles. As mammals are descended from organisms that followed a different evolutionary trajectory from the ancestors of birds beginning about 320 million years ago, chickens are only extremely distantly related to mammals.
Chickens and other birds are classified as members of the maniraptor clade based on fossil evidence. Maniraptors were two-legged, feathered dinosaurs with a unique wrist bone that enabled them to move their front limbs quickly and forcefully. This movement propels flying in birds.
Because birds have a warm blooded constitution as opposed to reptiles, which are described as cold-blooded animals with scales, birds were not classified as reptiles in earlier scientific classification methods. Despite these variations, birds have a lot in common with other living reptile species, including the capacity for egg production. Crocodiles and birds are the two groups of animals that are now alive that are most closely linked to one another.