Researchers have uncovered a major clue to a tantalizing and long-standing mystery: how migratory songbirds sense geomagnetic fields and use this information to orient their flight. The new findings document a specific "head-scanning" behavior, previously unappreciated, that is performed by the birds just prior to orienting their bodies toward their migratory direction.
Night-migratory songbirds are known to use a magnetic compass to guide them during yearly journeys covering thousands of kilometers, but how these birds detect the reference direction provided by the geomagnetic field, and where the sensory organ for this task is located, has remained unknown.
In the new work, Dr. Henrik Mouritsen and colleagues discovered that caged migrants - in this case, garden warblers - use movements called head scans to detect the direction of the Earth's magnetic field. In the natural geomagnetic field, the birds move toward their migratory direction immediately after performing a head scan. When the Earth's magnetic field is experimentally removed, birds move in a random direction immediately after performing a head scan. In addition, birds experiencing a zero magnetic field appear to more diligently search for the non-existing reference direction by tripling their head-scanning.
The findings of Mouritsen et al. confirm that the magnetic sensory organ must be located in the birds' head, and the fact that head-scanning frequency increases significantly when birds cannot detect the field can be used as a new method to determine the limits of the birds' magnetic sense: the researchers can expose the birds to a specific magnetic field and ask the birds whether they can sense the field. If they cannot sense it, they will "tell" the researchers by shaking their head more intensely.
Henrik Mouritsen, Gesa Feenders, Miriam Liedvogel, and Wiebke Kropp: "Migratory Birds Use Head Scans to Detect the Direction of the Earth's Magnetic Field"
Publishing in Current Biology, Volume 14, Number 21, November 9, 2004, pages 1946-1949.