Mechanisms of the light-dependent magnetic compass and magnetoreception in birds

During my PhD in Lund, I studied the orientation of European robins under different wavelengths and intensities of light (Muheim, Bäckman and Åkesson, 2002). With two Xenon Arc lamps and interference filters we produced light of very narrow half bandwidths (9-11 nm) and tested the magnetic compass orientation of juvenile birds exposed to 560.5 nm (green), 567.5 nm (green-yellow) and 617 nm (red) light at three different intensities (1, 5 and 10 mW m-2). Our wavelength ranges were much narrower than the ones used in previous orientation studies with birds performed under monochromatic light (half bandwidths in other studies range between 30 and 70 nm). Thus, we could in more detail examine the function of the magnetoreception mechanism in the expected transition zone between oriented and disoriented behaviour around 565 nm light under three different intensities.

We showed that (1) European robins are oriented in the seasonally appropriate migratory directions under 560.5 nm light, (2) they are completely disoriented under 567.5 nm light under a broad range of intensities, thus the transition between oriented and disoriented behaviour occurs in a very narrow zone of wavelengths, (3) they are able to orient under 617 nm light of lower intensities, though into a direction shifted relative to the expected migratory one, and (4) that magnetoreception is intensity dependent, leading to disorientation under higher intensities. Our results support the hypothesis that birds possess a light-dependent magnetoreception system based on magnetically sensitive, antagonistically interacting spectral mechanisms, with at least one high-sensitive short- and one low-sensitive long-wavelength mechanism.

A magnetoreception system with two magnetically sensitive, antagonistically interacting spectral mechanisms. The spectral mechanism in the shorter wavelengths (green) is more sensitive and directs the birds into the seasonally expected migratory direction, the low-sensitive mechanism in the long wavelengths (red) is less sensitive and directs the birds into a shifted direction relative to the species-specific migration direction. At 567.5 nm both mechanisms are excited equally which results in disorientation.

This model is also congruent with the observed reactions of birds to high-intensity lights, leading to axial responses or seasonally independent, "fixed" directions. A light-dependent magnetoreception mechanism working in a similar way as vision will be expected to show adaptation (saturation) when over-exposed to specific wavelengths of light. Saturation of one spectral mechanism would result in this mechanism being reduced in sensitivity and not responding or responding much weaker when exposed to light. Lowering the sensitivity of one mechanism could result in disorientation if the response in the two mechanisms became more or less equal, as we observed under 567.5 nm in our robins. Inclination compass orientation would thus temporarily not be possible anymore and the birds would have to resort to alternative orientation cues, or they would become disoriented.

Last updated: 11/11/2007