Light-dependent magnetoreception in birds: does directional information change with light intensity?

Magnetic compass orientation in birds is based on light-dependent processes, with magnetoreception being possible only under light containing blue and green wavelengths. To look for possible intensity-dependent effects we tested Australian silvereyes during autumn migration under monochromatic green light (565 nm) produced by light-emitting diodes at various light levels. At intensities of 0.0021 and 0.0075 W/m(2), the birds showed normal activity and were oriented in their seasonally appropriate migratory direction. Under low light of 0.0002 W/m(2) the birds were less active; scatter increased, but they still oriented in their migratory direction. Under a high light level of 0.0150 W/m(2), however, the test birds showed a counterclockwise shift in direction, preferring west-northwest instead of north. This change in behavior may reflect a change in the output of the magnetoreception system, resulting from a disruption of the natural balance between the wavelengths of light.