Welll, the further back in the formation you are, the less energy is needed to fly. Each bird aligns itself to take advantage of the air turbulence created by the bird in front of it. Kind of like how race car drivers draft each other. This effect is amplified as you progress toward the end. Lead birds (high social females) will take a break and drop to the rear so they can do more gliding and rest their tired wings on those long flights.