The bird in the lead (at the forward point of the `V' formation) works the hardest by being the first to `break through' the air, which offers resistance to its flight.
Just as a boat leaves a V-shaped wake of smoother water behind it, the lead bird leaves a V-shaped wake of slightly `smoother' air behind it (actually the lead bird creates a trail of air turbulence that helps lift along the V-shaped direction), and it is a bit easier for the other birds to fly in the wake of the lead bird.
If you watch a V-formation carefully, you'll notice that the lead bird does not stay in that position for very long and will drop back into the formation, while another, not-as-tired bird takes the lead, breaking through the air first.
Source : The Hindu
