By Tim Hirsch
BBC environment correspondent
US scientists believe they have made an important breakthrough in the mystery of how migrating birds manage to navigate thousands of kilometres and arrive at exactly the same spot each year.
A grey-cheeked thrush: Migrates through the US Midwest
Laboratory experiments in the past have suggested the birds may use a number of cues, including sunlight, stars and the Earth's magnetic field, which they can detect inside their bodies.
But this new study, reported in the journal Science, involved tracking a group of thrushes across hundreds of miles in the American Midwest and finding out what happened when they were deliberately confused by a man-made magnet and knocked off course.
The birds were captured during their northward migration and released at nightfall when they would normally continue their journey - having been fitted with tiny radio transmitters.
But just before setting off, the birds were exposed to the artificial magnetic field pointing east.
Their flight path was then tracked by researchers chasing animals for six days in a 1982 Oldsmobile with an antenna mounted on top.
On some nights, the scientists were held up by traffic police who were suspicious of the battered car zooming along remote roads laden with electronic equipment.
But the experiment yielded intriguing results. On the first night, the songbirds headed west, apparently knocked off course by the magnet.
On subsequent nights, they managed to correct this mistake and continued northwards towards their destination.
The researchers concluded that each night the thrushes must have "recalibrated" their inbuilt compass from the position of the setting Sun.
This helps to explain two enigmas surrounding the "flying by compass" theory.
First, the position of the magnetic North Pole shifts significantly each year, so magnetic signals alone would not be reliable, especially in higher latitudes.
A Swainson's thrush: Gets back on track at dusk
Also, bird compasses cannot distinguish between north and south, so you would expect them to be confused when crossing the equator.
Amazingly then, this study does seem to confirm that birds can "reset" their navigation systems daily by comparing the direction of the sunset with the magnetic signals they detect.
The research was conducted by William Cochran, at the Illinois Natural History Survey in Champaign, and colleagues.