Common Cranes (Grus grus) fly with their necks and legs stretched out. In migratory flight, cranes generally adopt a V formation. They regularly break this formation to gain altitude when warm air rises. They then return to their original direction of flight but in a slightly looser configuration. The flapping of the wings creates a small disturbance behind the bird. A sort of micro vortex rising and then falling according to the rhythm of the wings. The cranes line up behind each other and benefit from the rising air thus created, which saves their strength.