Magnetic orientation in birds: non-compass responses under monochromatic light of increased intensity

Migratory Australian silvereyes ('Zosterops lateralis') were tested under monochromatic light at wavelengths of 424 nm blue and 565 nm green. At a low light level of 7 x 10¹⁵ quanta m⁻² s⁻¹ in the local geomagnetic field, the birds preferred their seasonally appropriate southern migratory direction under both wavelengths. Their reversal of headings when the vertical component of the magnetic field was inverted indicated normal use of the avian inclination compass. A higher light intensity of 43 x 10¹⁵ quanta m⁻² s⁻¹ , however, caused a fundamental change in behaviour: under bright blue, the silvereyes showed an axial tendency along the east–west axis; under bright green, they showed a unimodal preference of a west–northwesterly direction that followed a shift in magnetic north, but was not reversed by inverting the vertical component of the magnetic field. Hence it is not based on the inclination compass. The change in behaviour at higher light intensities suggests a complex interaction between at least two receptors. The polar nature of the response under bright green cannot be explained by the current models of light-dependent magnetoreception and will lead to new considerations on these receptive processes.