Bird and animals have complex body structures . Mammals for performing work and birds for flying require large amount of energy which they get from the breakdown of glucose with oxygen . If there is no seperation between the oxygenated and deoxygenated blood then the requirement of energy wouldn't be fulfilled . Also bodies of mammals and birds gets heat up very quickly , to maintain the temperature it becomes necessary to separate oxygenared and deoxygenated blood in the circulatory system. Hence birds and mammals have double circulatory system that helps them to do so .
