Here is illustrating two ways of circulation system used by birds in their migration travel.
One is breathing. Birds fly at higher altitude and over high mountains sometimes during
their migration where oxygen content is relatively low in these cases. Birds have a specific
organ which can store oxygen in the normal environment and consume it in maximum
efficiency during their migration flying.
Another way is to use their muscles. Long flying make birds tired. At this stage, they use
a chest muscle to provide extra energy so as to reduce tireness.