Are birds mammals? Why, or why not?

No, birds are not mammals. Birds and mammals are two distinct classes of vertebrate animals with several key differences that set them apart:

Body Temperature Regulation:

Mammals are warm-blooded, which means they can regulate their body temperature internally and maintain a relatively constant temperature regardless of their environment.
Birds are also warm-blooded, just like mammals. They have a high metabolic rate that allows them to maintain a constant body temperature, typically higher than that of most mammals.
Reproduction:

Mammals give birth to live offspring (with a few exceptions like monotremes, which lay eggs), and they typically nourish their young with milk produced by mammary glands.
Birds lay eggs, which are externally fertilized. They do not produce milk but provide food for their offspring by regurgitating partially digested food into their chicks' mouths.
Skeletal System:

Mammals have a unique bone structure in the middle ear, including three small bones known as the ossicles (hammer, anvil, and stirrup). These bones are involved in hearing and are characteristic of mammals.
Birds also have a unique adaptation for hearing, but it involves a single bone in their middle ear called the columella. This bone is distinct from the mammalian ossicles.
Skin and Coverings:

Mammals typically have hair or fur covering their bodies. Some aquatic mammals, like whales and dolphins, may have very little or no hair.
Birds have feathers that cover their bodies. Feathers serve various functions, including insulation, protection, and aiding in flight.
Respiratory System:

Mammals have a diaphragm that helps in breathing, which is essential for regulating the flow of air into the lungs.
Birds have air sacs and a highly efficient respiratory system that allows for a continuous flow of air through their lungs, ensuring a constant supply of oxygen during both inhalation and exhalation.
In summary, birds belong to the class Aves, while mammals belong to the class Mammalia. While both birds and mammals are warm-blooded, they have distinct differences in terms of reproduction, skeletal structure, skin coverings, and respiratory systems that clearly differentiate them as separate classes within the animal kingdom.