Birds fly in a V-shaped formation to conserve energy and for better communication among the birds, according to Scientific American. Birds also fly in a J-shaped formation, and studies have shown that a true V-shaped formation is less common than a J-shaped formation.
In a V formation, birds take advantage of the up-wash vortex fields created by the wings of the birds in front. The formation also allows for careful orientation and collision avoidance. During short-distance flights, energy conservation is less important; on long-distance flights, such as migrations, an optimal position in the flock is vital for saving energy. The leaders of formations change from time to time, but scientists have not determined the causes, frequency and characteristics of these changes.