Why is it necessary to separate oxygenated and deoxygenated blood in mammals and birds?

Explanation

Both mammals and birds are warm blooded animals which means they can control their body temperature an do not have to depend on environment for their body temperature regulation. Because of this both birds and mammals require optimum oxidization of glucose which would be possible with good supply of oxygen. So, it is required to have separate oxygenated and deoxygenated blood to supply the required mount of oxygen.