Why do birds fly on V shape in the group while travelling?

The V Formation, also known as the Echelon formation, is very frequently seen in migratory birds such as geese, crane, stork, who migrate or travel over long distances.

This V shape formation of a flock is influenced by aerodynamics as in aircraft, in which the air surrounding the object gets displaced and produces lift.

That’s why the birds take this formation so that flying will become easier for them. 

Over the course of migration, one bird leads the front of V form, as the bird flap its wings, the air flowing off their wingtips gives birds in the back of the V form an extra lift, which reduces the amount of energy the birds need to fly. 

Now, the leading front bird alternately keeps changing so that no one bird gets too tired. This process helps birds to fly long distances without taking any rest.