Actually, a flock of birds will share the lead of the V and rotate throughout the migratory flight. Therefore, as the leaders tire, they fall further back in the flock and the birds behind will take over.
This indicates that the lead would change quite often during a very long flight. It means that all the birds flying in the flock get a chance to glide easier at the back of the flock. It’s a truly wonderful example of co-operation in action.