Question

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

Answer 1

Warm-blooded animals are mammals and birds. To regulate their body temperature, they continuously use energy. They have higher energy requirements and thus need more oxygen to create energy.

Complete answer:

Through cooling themselves when they are in a colder climate and by warming their bodies when they are in a colder climate, warm-blooded animals such as birds and mammals retain constant body temperature.
There is a double circulating system in birds and mammals where both the oxygenated and deoxygenated blood is kept separate.
Thus, for more cellular respiration, these animals need more oxygen so that they can generate more energy to sustain their body temperature.
If the oxygenated blood remains different, it is even safer, as its combination with deoxygenated blood would impure the whole blood.
They also need to separate oxygenated and deoxygenated blood so that their circulatory system is more effective and that their body temperature can be controlled continuously.

Additional information:
In the circulation, 'oxygenated' blood receives oxygen from the lungs by binding it to hemoglobin.
'De-oxygenated' blood returns to the lungs with oxygen-depleted haemoglobin. The blood would also transport the resulting carbon dioxide for exhalation back to the lungs.
Oxygenated blood has a significant proportion of red blood cells bearing oxygen on the haemoglobin De Oxygenated blood has less oxygen and more carbon dioxide being borne by the haemoglobin.
Oxygenated blood: lung $\to$ left chambers of the heart $\to$ body
Deoxygenated blood: body $\to$ right chambers of the heart $\to$ lung

Note: In the lungs, by absorption, the pulmonary capillaries absorb oxygen. The lungs have a lower oxygen content than the lungs, so oxygen diffuses through the blood through the thin membrane of the capillaries. This causes oxygenation to occur in the blood.