The bar tailed Godwit is about 37/39cms (15ins) long which summers in the arctic yet migrates to warmer locations during the winter. These include astonishing non-stop flights over the Pacific to New Zealand every Northern winter.
As you can see from the tracks, you will see it flies in a perfect straight flight from the Arctic to New Zealand.
Most literature on animal navigation considers that birds navigate using magnetic cues or by smell.
The Godwit and the Arctic Terns who do the same trick of flying from one end of the world to the other. In my opinion, they cannot possibly use these cues. Even using the sun does not work that well either for these sort of journeys.
When the Godwits leave the Arctic the skies are usually completely overcast and can remain like that for weeks on end. Compasses at these latitudes face vertically down and give no sense of direction.
So, I believe that the Godwits have an inbuilt sense of direction; they just know where they have to go. Many humans have a sense of direction too, they too can just sense the way to go too.
Why do I think this? Well, if you have a sense of direction you know the way you need to go and you can make adjustments for the right direction whatever is happening to you. In the tracks taken by the Bar-tailed Godwit you will see that the flights are straight. The birds are continuously adjusting to keep the most efficient straight track to get to their destination.
Amazingly they can clearly make continuous adjustments for winds. Making these adjustments was always a huge problem for early flyers.
This sense of direction is well known to those humans that have it. My contention is that the birds have the same sense of direction and use the same mechanisms as I have explained that dowsers use.