Wiltschko et al. reply

Abstract

Replying to C. M. Hein, S. Engels, D. Kishkinev & H. Mouritsen Nature 471, doi:10.1038/nature09875 (2011)

Hein and colleagues¹ challenge our 2002 paper², claiming that they cannot replicate our findings. The paper had two conclusions: (1) magnetic compass information is mediated by the eyes, as had been proposed by Ritz and colleagues³, and (2) the magnetic compass is lateralized in favour of the right eye. The new data do not contradict the first conclusion; in fact, this has been supported by a recent paper from the authors⁴. It is only the second conclusion they question, although it has been demonstrated not only in two species of migrants^2,5, but also in domestic chickens⁶ and is the basis of a new paper indicating an interaction between contour vision and magnetoreception⁷.

Main

These obvious differences in findings require explanations, and offhand, three possibilities come to mind:

(1) The authors do not observe migratory orientation, but a ‘fixed direction’ response. ‘Fixed direction’ responses do not involve the inclination compass based on the radical pair mechanism, but are polar responses originating in the magnetite-based receptors in the beak⁸; they are not lateralized⁹. The observed scatter is in agreement with this interpretation, as ‘fixed directions’ are often more scattered than compass responses⁸. Critical tests to distinguish between the two types of responses, like inverting the vertical component of the magnetic field, are missing.

(2) The studies by Hein et al.¹⁰ were autumn experiments, where young birds fly innate compass courses¹¹, whereas ours^2,5,7,8 involved spring experiments, where birds can use true navigation to head back to the familiar breeding regions¹². There are indications that the navigational ‘map’ is lateralized in favour of the right eye/left brain system¹³, which, in turn, could have led to a lateralized response.

(3) Another difference between the studies is the number of tests per bird. Whereas we tested the birds two^2,5 or three^7,9 times, the authors’ means are based “on 4.11 ± 2.76 (s.d.) active and oriented tests per condition”, which implies that the individual birds have been tested more often. Hence the total time the birds had their right eye covered was considerably longer than in our studies. In certain tasks acquired unihemispherically, an interhemispheric transfer is observed in animals that have to rely on the naive eye; in some cases, this takes just a few hours¹⁴. A similar transfer may have occurred when the right eye was covered for a longer period. The observation that the vectors of the birds with the right eye covered are the shortest in both magnetic conditions is in agreement with this interpretation. This could also explain the weaker lateralization observed in pigeons¹⁵, where the total time of covering the right eye was also much longer. It would mean that although the avian magnetic compass is normally mediated by the right eye only, left-eye input is able to substitute the process after a critical amount of time.

In summary, there are considerable differences between the studies. Which of them or which possible combination of them caused the difference in findings cannot be decided at present, but will be determined by future experiments.