This diagram show the evolutionary and structural relationship of the cerculation system between Amphibian, Modern Reptile, Crocodile, which are not as efficient as Birds (right side -aorta) and Mammals (left side-aorta)
Since birds are warm blooded, they
have a higher metabolism than reptiles. The maintenance of a high body
temperature is necessary for flight, for it requires a great input of oxygen
and energy, over a long period of time. For birds, the great arch of the
aorta, which carries the blood from the heart to the body, passes over
and down the right side of the chest, rather than the left, as it is in
ourselves and in all other mammals. The modern bird circulatory system
is comparable to mammals, if not most efficient. A sparrow's heart is nearly
three times heavier, compared to body weight, than that of a mouse. The
diagram below shows the breathing system of a bird, showing the location
of the air sacs.