Birds are not mammals; birds belong to the class Aves and are more closely related to reptiles than to mammals. Mammals belong to the class Mammalia. Birds are the only living animals to have feathers, whereas mammals are the only animals to have hair.
All living birds are directly descended from dinosaurs; a bird is far more closely related to an alligator than it is to a bat or any other mammal!
Although both birds and mammals do share a common ancestor, the ancestors of birds split from the ancestors of mammals over three hundred million years ago.
With such a vast amount of time (and evolution) separating birds and mammals, the answer to the question “Is a bird a mammal?” is definitely “no!”.
To fully understand why a bird is not a mammal, we need to take a closer look at both groups of animals…