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 tes ted 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 activit y and were oriented in their seasonally appropriate migratory direction. Un der low light of 0.0002 W/m(2) the birds were less active; scatter increase d, 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 sys tem, resulting from a disruption of the natural balance between the wavelen gths of light.