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Quantum Compass of Migratory Birds

FIGURE 4.5 Wavelength dependency of the avian magnetic compass (Wiltschko et al., 2011, data from Wiltschko

et al., 1993, 2007, 2010; Wiltschko and Wiltschko, 1998; Rappl et al., 2000; Muheim et al., 2002 and unpublished).

Above: spectra of the light-emitting diodes used in the tests; below: orientation of fve bird species tested, with + indi­

cation oriented behavior and Θ indicating disorientation. (Reproduced with permission from Wiltschko et al., 2011,

Copyright 2011, Elsevier.)

568 nm (Muheim et al., 2002). Tus, it requires short-wavelength light from UV to green (Wiltschko

and Wiltschko, 2014). Tis pattern seems to be common to passerine species (Rappl et al., 2000), homing

pigeons (Wiltschko and Wiltschko, 1998), and domestic chickens (Wiltschko et al., 2007).

Te experiments mentioned above used low-intensity monochromatic light of a quantal fux of about

7 × 10¹⁵ quanta/s m² as found 45 minutes before sunrise and afer sunset (only with UV, the intensity

was about 0.7 × 10¹⁵ quanta/s m², i.e., 1/10). Under monochromatic light of higher intensities and under

bichromatic light, migratory birds no longer prefer their migratory direction (Wiltschko et al., 2010).

However, birds are able to use their magnetic compass also under high light levels, provided the light is

“white,” i.e., composed of a wide variety of diferent wavelengths—the magnetic compass can be used,

e.g., by homing pigeons in bright daylight.

Amphibians and reptiles were found to use an inclination compass-like birds, but that of amphibians

shows a diferent wavelength dependency (Phillips, 1986a, b), and that of marine turtles does not require

light at all (Light et al., 1993; Lohmann and Lohmann, 1993). In contrast to the amphibians, the spectral

range where birds obtain normal magnetic compass information includes the larger part of the visual

spectrum. At the same time, this wavelength dependency of magnetoreception shows no relationship to

the peaks of the four color cones of the birds’ visual system (Maier, 1992), and thus speaks against their

involvement in mediating magnetic directions, suggesting the existence of another type of receptor. Te

birds’ response looked like an “all-or-none”-response that could be attributed to one receptor, yet the

rather abrupt transition to disorientation, which persisted under the increased intensity of the yellow

or red light (Wiltschko and Wiltschko, 2001; Wiltschko et al., 2004), seems to suggest an antagonistic

interaction with a second receptor (Wiltschko and Wiltschko, 2005). When the eyes were illuminated

with monochromatic light of various wavelengths, units with a peak of responsiveness around 503 nm

and others with a peak beyond 580 nm were identifed, thus suggesting the two types of receptors with

diferent absorption maxima, a fnding that is in agreement with the behavioral studies likewise indicat­

ing two types of receptors with absorption peaks in the blue-to-green and in the long-wavelength range

(e.g., Möller et al., 2001; Wiltschko et al., 2004).