A group of scientists has released a study that confirms that birds migrate using the earth's magnetic field. (There may well be other influences)
Put simply, Eurasion Reed warblers migrate thousands of miles. After one successful migration, they can repeat the trip.
The "how" was the question. The proof came when they trapped some warblers, then, when they were released, they noticed that the feet had scratched the trap tubes. All the birds chose to leave the tubes in the direction of their breeding grounds. (Wow)
They set up a chamber where they could manipulate the magnetic field. Basically gave them a false "true north" which the birds then followed.
Birds appear to have a Cartesion map. (Like gps) North to South by magnetic strength and east to west by declination.*(An example would be the angle the sun's rays hit the earth 2/16)
So the data that you brave birdwatchers are recording may be used in future understanding of migration. Those cool roost and feed observations during northward migration may further our knowledge base.
[Other research is needed to determine what parts of the bird work this magic.]