Why hearts of mammals and birds have the left and right sides separated?

Mammals and birds both have a four-chambered heart. These creatures have complex metabolism and are warm-blooded. These conditions required a constant flow of oxygen. However, oxygenated and deoxygenated blood can mix in the heart's chambers if there is improper separation, which can be problematic for these complex animals. Hence, the left and right sides of the heart are separated in birds and mammals