This link is a very complete overview of the animal navigation at present in 2015. It covers all the important work that has been done and discusses what has been found along with all the problems associated with different approaches.
As you will see it ends up by saying that we still do not know how animals navigate. This is of course our stance at www.animalnav.org.
We are still fascinated by the fledgling cuckoo problem: how do they inherit the instructions of how to get to their wintering grounds in the Congo?