The anatomical and physiological features birds and mammals have in common are possessing a spinal cord and vertebral column, four-chambered hearts and warm-bloodedness. Birds and mammals also both generally invest a great deal of time and energy into raising their young, and are energetic and active consumers. Both birds and mammals have flying, fully terrestrial and amphibious members, although mammals have a wider range of lifestyles.
Birds and mammals are the two major groups of warm-blooded animals on Earth, although a few fish such as tuna also are capable of internally regulating their temperature. A few species of mammals, such as mole rats, have little ability to regulate their temperature, but they are the minority of mammal species. Both mammals and birds evolved from reptiles, although most scientists believe that birds evolved from dinosaurs, while mammals came from earlier reptiles.
For all that birds and mammals have in common, however, they are not closely related and have many differences. Mammals have well-developed teeth, while birds have no teeth at all, having evolved their lighter beaks to replace them. The red blood cells of birds have nuclei, while mammals have red blood cells with no nuclei. Mammals generally give birth to live young and feed them milk, while birds lay eggs and must feed their young food items directly.