The answer is ingenious and beautiful. A human-powered microlite guides them on their first migration and they are able to find their way back. In subsequent years their navigation improves as the learning process consolidates.
In San Diego, earlier this year, I watched pelicans flying home in their shifting V formations. They gave out had aura of control, power and even wisdom. To see that the cranes are able to trust the human navigator for the first trip is very moving.
Science, 30 August 2013, pp. 999-1002.