What is a bird if is not a mammal?

Answer 1

it depends on what is your definition of bird. Mammals are warm blooded animals that keep their bodies warm with the heat produced from there metabolic reactions, so do birds. Dinosaurs have been found with feathers, does that make them a bird or reptile?

Reptiles lay eggs as do birds (and some mammals).

To clearly answer your question, birds are simply another class (aves) in the animal kingdom, just like fishes and amphibians. There are some theories as of how birds came about (the most realistic one, is that reptiles gave mammals and birds) because some reptiles (dinosaurs have been found with feathers (missing link between reptiles and birds) and some other reptiles have been found with mammal glands (missing link between reptiles and mammals).

Answer 2

There are many classes of species recognized today. Birds are in the class "Aves." Reptiles (crocodiles, turtles, snakes, lizards), are in the class "Reptilia."
Anything you would consider a mammal is under the class "Mammalia,"
insects "Insectia,"
amphibians "Amphibia,"
Some only contain a small number of organisms and are less well known, such as "Myxini," which only contains hagfish. There are many, many more. Life on earth is amazingly hard to classify!

Answer 3

A bird is not a mammal. Birds do not give birth to already living young, they fertilize and lay eggs. Also, birds do not feed their young milk (when they hatch) which is another primary trait that must be present in order for them to be classified as a mammal.