Lighter colored muscles are used for quick explosive flight. Consider a roast chicken. There is light and dark meat. Game birds have breasts of light colored muscle while birds needing longer flights have breast muscles that are dark. Some birds, such as pigeons, have breasts with a mixture of the two types of muscles, allowing them to quickly get away but also cover great distances.
Birds lungs are much different than human lungs.
Their lungs are connected to a series of air sacs that are throughout the body and into even the breast muscles. Each breath is specially processed to extract much more oxygen per breath than our own lungs are capable of doing. Remember, flying is more demanding than even running. And this system allows them to get enough oxygen even at high altitudes, migrating over mountain ranges. Bird hearts are relatively larger and stronger than mammals.