Why do birds fly in formation shaped like the alphabet 'V'?Which bird works the hardest in such a formation and why?

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(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 a very long and will drop back into the formation,while another,not as tired bird takes the lead,breaking through the air first.