Most everyone is aware of at least some of the differences between birds and mammals. Whereas birds have feathers, lack teeth and lay eggs, mammals have fur or hair for insulation, possess teeth and give birth to live young. Although birds are more closely related to reptiles than mammals, birds and mammals have several characteristics in common.

Both birds and mammals are warm-blooded, which means they can maintain a constant body temperature and do not need to rely on an external heat source to stay warm. This similarity lends itself to several other commonalities, such as similar caloric requirements by weight and the ability to remain active in colder temperatures. Cold-blooded animals, such as reptiles, do not have to eat as much, but they also cannot survive colder temperatures. Being warm-blooded also gives birds and mammals the unique ability to live on any landmass on Earth.

All mammal and bird species are classified as vertebrates, meaning they have backbones and skeletal systems made of bone. Birds, however, have hollow bones with a crisscrossed matrix for added strength. The hollow bones are lightweight, which allows the bird to take flight, while the structural matrix adds strength to withstand the pressure of taking off and landing.

Birds require a lot of energy in order to fly. This also necessitates a circulatory system that is both efficient and effective, so they have evolved a four-chambered heart with two atria and two ventricles, just like mammals. One of the main benefits of this type of circulatory system is that it allows the separation of oxygenated and deoxygenated blood. Other species of animals, such as reptiles, have hearts with fewer chambers, which means both oxygenated and deoxygenated blood have to travel through some of the same chambers — a much less efficient model.

The blood of birds and mammals contains both red and white blood cells, called erythrocytes and leukocytes respectively. The red blood cells in both classes of animals contain hemoglobin, a protein containing iron that is responsible for oxygen transport and gives blood its red color. While both classes have erythrocytes, mammals' erythrocytes lack a nucleus, while the erythrocytes of birds do have nuclei. The leukocytes of both classes function in immune regulation.

Another similarity between birds and mammals is that both classes care for their young after they're hatched or born. The length of time varies from species to species, depending on the age that the young are first able to take care of themselves. Female mammals feed their young by lactating, while feed their young beak to beak.