Which bird works the hardest in formation like V-shaped alphabet during flight of birds 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, the lead bird leaves a V-shaped wake of '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 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.