The most accepted reason is that the flight formation plays an aerodynamic role. It conserves the energy of birds. Each bird flies slightly above the bird in front of him, resulting in a reduction of wind resistance. The birds take turns being in the front, falling back when they get tired. In this way, the birds can fly for a long time before they must stop for rest especially during migration. This formation also enables visual communication among members of the flock and reduces the possibility of collisions and of birds becoming separated from the flock.