What advantage do many birds gain by flying in V-formation?
As a bird flap its wings it disturbs the air and leaves whirling eddies behind. Some gregarious species take advantage of the upward sections of these whirls and each bird in the V-formation stations itself at the correct place so the inner wing obtains support from the wake of the bird immediately ahead. Thus every bird in the flock except the leader saves energy by using the V-formation type of flight.
For more info see:1001 Questions Answered about Birds
