The orientation responses observed under UV, Blue, Turquoise and Green light at the various intensities clearly fall into two distinct categories: under the lowest light level, which was 0.8·1015 quanta s-1 m-2 for UV and 8·1015 quanta s-1 m-2 for Blue, Turquoise and Green, the robins showed a strong preference for their migratory direction. Under 8·1015 quanta s-1 m-2 UV light and under increased light intensities of the other colors, a variety of responses, mostly axial preferences, was observed. The northerly headings observed under bright Turquoise, although superficially similar to the migratory direction, also represent responses of a different nature  (see below). Together, the data clearly show that even when the birds were no longer heading in their migratory direction, their behavior continued to change as the light intensity increased. This implies that it is still controlled by light-dependent processes.
The following considerations will focus on (i) the intensity-dependent pattern of responses indicated, (ii) possible reasons for the change in the nature of the responses, (iii) the possible involvement of the color cones and (iv) the origin of directional information for the axial and 'fixed direction' responses.
Compass orientation and other responses
The behavior under the low light intensity Blue, Turquoise and Green represent true migratory orientation: under this and similar light levels, birds prefer their migratory direction in spring as well as in autumn. They showed the expected seasonal reversal, and the compass mechanism involved is the normal avian inclination compass [4, 7, 16]. Tests with oscillating magnetic fields indicate that magnetoreception under Green and Turquoise light is based on radical pair processes [2–4] as proposed by the model of Ritz and colleagues . The same can be assumed for dim Blue light, where the orientation corresponds to that under Green and Turquoise in all other aspects , and presumably also for UV light of very low intensity. That is, under low monochromatic lights from 373 nm UV to 565 nm Green, magnetoreception works in the normal way as under 'white' light, providing birds with directional information from the magnetic field that they can use to locate their migratory direction and probably also any other direction they may wish to pursue.
The behavior under monochromatic light of higher intensity is different. Increased monochromatic lights do not simply cause a switch from migratory orientation to another specific response; instead, they elicit a variety of different responses. This is most conspicuous under green light: Here, the birds show disorientation at 36·1015 quanta s-1 m-2, then, as intensity increases, a preference for the east-west axis and finally a preference for the north-south axis. Muheim and colleagues , also testing robins under green light, observed axial behavior in the migratory direction and in the opposite direction under intensities of 14 and 29·1015 quanta s-1 m-2, which may be a first step away from normal migratory orientation.
Together, the responses observed under 8·1015 quanta s-1 m-2 UV and at higher intensities of Blue, Turquoise and Green suggest that the pattern observed under Green might be a general one. Under all wavelengths, we found a preference of the east-west axis that, under increased intensity, was followed by a preference of the north-south axis, with the modification that under Turquoise, a unimodal 'fixed' northerly direction  replaces the axial north-south tendency. However, where normal migratory orientation ends and random and axial behavior begins appear to depend on the wavelength as well as on the intensity of light.
The different types of responses at brighter light imply a disrupted function of the magnetoreception system under monochromatic light of higher intensity. It raises a number of questions: What causes the magnetoreception system to cease functioning in the normal way? Is there a functional significance of the axial and 'fixed' direction responses? And: what is the nature of the directional information for the 'fixed' directions and axial responses?
What causes the change in magnetoreception?
An effect of the brighter monochromatic lights on circadian patterns and motivation (e.g. [19, 20]) is rather unlikely in view of the fact that even the brightest lights used in the present study were of intensities found well after sunset (see Method section), i.e., at a time of day when nocturnal migration is in progress. Also, the disruption of the normal magnetic perception process cannot be attributed to the higher intensity of light itself or to saturation of the crucial receptors. The avian magnetic compass works under bright sun light (see e.g. [21–24] for homing pigeons and day migrants), and cage tests showed that also nocturnal migrants use their magnetic compass for migratory orientation under natural day light when migratory behavior was induced by food deprivation . Caged robins, too, were well oriented under 'white' test lights of higher intensity . However, the 'white' test lights, like day light, were composed of wavelengths from all parts of the spectrum. Therefore, it seems to be the narrow bandwidth of the monochromatic test lights used rather than their brightness that gives rise to the observed effects.
The same wavelengths of light allow very good orientation at low intensities, but disrupt the magnetic compass orientation as the intensity of the monochromatic light increases. Our experiments were performed under light levels that, in humans, are mesopic conditions, i.e., where both the rod and the cone system is active. In humans, this transition zone covers at least 3 log units ; its extension in birds is unknown. Note that the light levels where we observed a change from compass orientation to an axial response along the east-west axis increased with increasing wavelengths, from UV over Blue and Turquoise to Green, suggesting a similar relationship for the end of normal perception of magnetic directions. This is a striking parallel to the sensitivity of the color cones, which decreases with increasing wavelength, with the UV-sensitive-cone type being more sensitive than the short-wavelength-sensitive cone, this type being more sensitive than the medial-wavelength-sensitive cone and the long-wavelength-sensitive cone type being least sensitive [see e.g. ]. This implies an involvement of the color cones under the higher light intensities, suggesting that the perception of magnetic directions works properly under monochromatic light as long as the test lights do not activate the cones above a certain level.
A possible role of the color cones?
The radical pair model  proposes that photon absorption causes a photopigment to form radical pairs and generate the signals that mediate magnetic compass information. However, at present, neither the nature of the relevant photopigment nor the type of cells where the reception processes take place are precisely known. A role of the rods and color cones in avian magnetoreception is usually not considered, because rhodopsin and the other opsins do not form the required radical pairs. Ritz et al.  therefore proposed that these could be formed by cryptochrome, a novel photopigment first known from plants (see ), but also found in the retina of chickens  and of passerine birds [30, 31]. Cryptochromes have recently been shown to mediate magnetic effects in plants . This implies the possibility of specialized photoreceptors for magnetoreception. The disruptive effect of intense monochromatic light, on the other hand, suggests that these receptors may interact with the normal visual perception system, in some complex way.
One possibility is that cryptochrome does not directly absorb the light, but receives the energy from a light-harvesting system of other pigments, as it is proposed, e.g., for photosynthesis . Yet under this assumption, it is hard to explain why higher light intensities should lead to a change in the nature of response away from normal compass orientation to axial responses and 'fixed direction'.
However, the behavior suggesting an involvement of the cones is only observed under higher intensity monochromatic light. This has to be included in the considerations on the role of cones in magnetic perception. The answer may lie in the fact that the output of a given cone is affected by both, the wavelength of the incident light and its intensity. Both parameters together give only one output value. Color perception is then based on the balance of the outputs of the three (mammals) or four (birds) cone types, as it is measured, for example, by the retinal ganglion cells where the input from the photoreceptors converges. Natural light will always excite several types of cones, since even objects that appear unicolored to us usually reflect a multitude of different wavelengths. Hence all cone types normally receive at least a certain amount of excitation by photons. In view of this, it is quite conceivable that the color system is tuned to perceive a mixture of almost all visual wavelengths under normal conditions.
Monochromatic light would cause an imbalance between the different receptors, and there is a lot of evidence that a strong imbalance in the color of the visual scene lead to strong habituation of selected cones, which in turn causes the appearance of aftereffects like the sensation of the countercolor when the imbalance is eliminated . By using monochromatic light with only a narrow spectral band, but of a relatively high intensity, the difference between the excitation of the cones projecting to one opponent color ganglion cell might become too large to be accepted by the system as normal, and the ganglion cell will no longer produce the appropriate activity. This may cause the visual system to also reject the magnetic information because it could be erroneous. In other words, the visual system may be able to gate, i.e. control the transfer, of the magnetic input somewhere on its way to the brain area where it is processed. Although there is ample evidence for the existence of such gating systems – almost all sensory information, for example, is thought to be gated in the thalamic nuclei on its way to the forebrain  – this is a mere assumption in the case of magnetic information. The activation of a visual brain area only at night recently described  might be an example of a gating process that allows the transfer of information towards this area only under certain conditions.
Whether an imbalance between the different color receptors is the correct explanation for the responses observed under high intensity monochromatic light must remain open at present. It means, however, that these responses need not necessarily be of functional significance. Instead, they might be by-products of a perception system driven beyond its functional limits, reflecting a complex relationship between various receptors and units that awaits further analysis.
Where does the polar magnetic information originate?
The unimodal response at 54·1015 quanta s-1 m-2 Turquoise was found to be polar, not involving the normal avian inclination compass, and tests applying high frequency fields showed that it is not based on radical pair processes . It seems likely that the axial responses under intense monochromatic light share these characteristics. This raises the question where this type of directional information comes from, if it does not originate in radical pair processes.
A magnetite-based receptor seems to be a logical assumption, as magnetite-based receptors could convey polar directions (see e.g. ). Magnetite has been found in the ethmoid region and in the upper beak of birds [38, 39], but electrophysiological recordings from the corresponding branch of the trigeminal nerve  as well as behavioral studies [41–43] seemed to suggest that magnetite-based receptors in birds provide information on magnetic intensity rather than directional information. However, it cannot be excluded that they additionally mediate directional information. The relationship between the axial preference and the orientation of magnetite particles described in salamanders  appears to suggest a role of magnetite in these responses, and a recent study  indicates that another 'fixed' direction response in birds was indeed mediated by the iron-based receptors in the upper beak .
Although a magnetite-based mechanism is certainly an option, it must be considered that the specific manifestation of the behavior observed under monochromatic light of higher intensity clearly depends on the intensity and wavelength of light. This is not only true for the present study, but also for various 'fixed' directions observed in other studies [7, 14, 17, 45]. The control of the axial and 'fixed' direction responses by the ambient light regime is difficult to explain by attributing it to a magnetite-based mechanism without auxiliary assumptions, like e.g. interaction between the magnetite-based and light-dependent mechanisms.