Birds fly in a V formation because it is more aerodynamic than flying in a scattered formation.
When a bird flies, the flapping of its wings disturbs the air, creating an upward current. A bird can use this “updraft” by positioning itself in just the right position behind another bird.
Thus it can take advantage of the air movement. The inner wing of the bird flying behind another bird receives support form the air, and flying becomes much easier and more efficient.