Night-migratory songbirds possess a magnetic compass in both eyes

Previous studies on European robins, Erithacus rubecula, and Australian silvereyes, Zosterops lateralis, had suggested that magnetic compass information is being processed only in the right eye and left brain hemisphere of migratory birds. However, recently it was demonstrated that both garden warblers, Sylvia borin, and European robins have a magnetic compass in both eyes. These results raise the question if the strong lateralization effect observed in earlier experiments might have arisen from artifacts or from differences in experimental conditions rather than reflecting a true all-or-none lateralization of the magnetic compass in European robins. Here we show that (1) European robins having only their left eye open can orient in their seasonally appropriate direction both during autumn and spring, i.e. there are no strong lateralization differences between the outward journey and the way home, that (2) their directional choices are based on the standard inclination compass as they are turned 180° when the inclination is reversed, and that (3) the capability to use the magnetic compass does not depend on monocular learning or intraocular transfer as it is already present in the first tests of the birds with only one eye open

Autumn orientation results if only the first two active and oriented tests per bird are considered.

<p>NMF  =  normal magnetic field; CMF  =  changed magnetic field; α  =  group mean direction; N  =  number of individuals; r  =  length of the mean vector; ns not significant; * p<0.05; ** p<0.01, significance by the Rayleigh test.</p

Birds can use their magnetic compass if light and/or visual input reaches any one eye.

<p>A–B: European robins equipped with eye covers with a hole in front of both eyes, C–D: birds equipped with eye covers allowing light and visual input to reach only the right eye, E–F: birds equipped with eye covers allowing light and visual input to reach only the left eye. The data in A, C, and E were collected in an unchanged magnetic field (NMF). The data in B, D, and F were collected in a magnetic field turned 120° counter clockwise (CMF). mN  =  magnetic North. For description of the circular diagrams, see legend to Fig. 2 (redrawn after Hein et al. <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0043271#pone.0043271-Hein2" target="_blank">[41]</a>).</p

Within-subject comparisons of the monocular orientation of European robins.

<p>Comparison of each of the monocular conditions with the binocular mean of each individual taken as a reference (i.e. taken as zero value, *). There is no systematic difference between the angular deviations of the right eye open condition (A 325°±32°, r = 0.51, p<0.001, N = 26; B 27°±26°, r = 0.60, p<0.001, N = 27) and the left eye open condition (C 333°±51°, r = 0.35, p<0.05, N = 27; D 310°, r = 0.13, p = 0.65, N = 26) in neither the normal magnetic field condition (NMF A, C), nor in the changed magnetic field condition (CMF B, D) For description of the circular diagrams, see legend to <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0043271#pone-0043271-g002" target="_blank">Fig. 2</a>.</p

Birds can use their magnetic compass in autumn and spring using only their left eye.

<p>European robins equipped with eye covers allowing light and visual input to reach only the left eye were tested in autumn (A, B) and spring (C, D). The data in A and C were collected in an unchanged magnetic field (NMF). The data in B and D were collected in a magnetic field turned 120° counter clockwise (CMF). A, B are redrawn after Hein et al. <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0043271#pone.0043271-Hein2" target="_blank">[41]</a>. mN  =  magnetic North. The arrows indicate the group mean vectors. The inner and outer dashed circles indicate the radius of the group mean vector needed for significance according to the Rayleigh Test (p<0.05 and p<0.01 respectively). The lines flanking the group mean vector indicate the 95% confidence intervals for the group mean direction.</p

Orientation results in autumn according to the age of the birds.

<p>NMF  =  normal magnetic field; CMF  =  changed magnetic field; α  =  group mean direction; N  =  number of individuals; r  =  length of the mean vector; ns not significant; * p<0.05; ** p<0.01; *** p<0.001, significance by the Rayleigh test.</p

Night-migratory garden warblers can orient with their magnetic compass using the left, the right or both eyes

Several studies have suggested that the magnetic compass of birds is located only in the right eye. However, here we show that night-migrating garden warblers (Sylvia borin) are able to perform magnetic compass orientation with both eyes open, with only the left eye open and with only the right eye open. We did not observe any clear lateralization of magnetic compass orientation behaviour in this migratory songbird, and, therefore, it seems that the suggested all-or-none lateralization of magnetic compass orientation towards the right eye only cannot be generalized to all birds, and that the answer to the question of whether magnetic compass orientation in birds is lateralized is probably not as simple as suggested previously