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Magnetic compass orientation in European robins is dependent on both wavelength and intensity of light

Rachel Muheim^*, Johan Bäckman and Susanne Åkesson

Bird Migration Group, Department of Animal Ecology, Lund University, Ecology Building, SE-223 62 Lund, Sweden

View larger version (29K): [in a new window]   Fig. 5. Summary of orientation experiments (in blue), homing experiments (in red) and neurophysiological studies (in black) examining light-dependent magnetoreception in birds, published until 2001. Also included are the results of the present study (in green). The graph shows the wavelengths and intensities under which birds were tested in orientation experiments and in an electrophysiological study (filled symbols indicate experiments revealing significant orientation, and open symbols indicate experiments revealing disorientation). The triangles indicate the average peak wavelength, and the horizontal bars indicate the half bandwidth (/2) of the spectrum, if available. Diamonds indicate oriented behaviour into directions significantly different from the seasonally expected migratory direction. Magnetic responsiveness of units of the basal optic root (nBOR) in the electrophysiological study on pigeons by Semm and Demaine (1986) is indicated by crosses (larger crosses indicate peak responses). The symbols refer to the following published orientation experiments: full-spectrum light at intensity M (Wiltschko et al., 1993; Wiltschko and Wiltschko, 1995; Munro et al., 1997; Rappl et al., 2000) and intensity XXL (Möller et al., 2001); blue light at intensity M (Wiltschko et al., 1993; Wiltschko and Wiltschko, 1999, 2001; Rappl et al., 2000), and intensity XL (Wiltschko and Wiltschko, 2001); turquoise light at intensity M and intensity XL (Wiltschko and Wiltschko, 2001); green light at intensity S (Wiltschko et al., 2000a), intensity M (Wiltschko et al., 1993, 2000a,b, 2001; Wiltschko and Wiltschko, 1995, 1999, 2001; Munro et al., 1997; Rappl et al., 2000), intensity L (Wiltschko et al., 2000a,b) and intensity XL (Wiltschko et al., 2000a,b; Wiltschko and Wiltschko, 2001); yellow light at intensity M (Wiltschko et al., 1993; Wiltschko and Wiltschko, 1999, 2001; Rappl et al., 2000) and intensity XL (Wiltschko and Wiltschko, 2001); and red light at intensity M (Wiltschko et al., 1993; Wiltschko and Wiltschko, 1995; Munro et al., 1997; Rappl et al., 2000; Möller et al., 2001). The asterisk refers to the study by Möller et al. (2001) where significant mean orientation was found after pre-exposure to red light only. The homing experiments refer to those of Wiltschko and Wiltschko (1998).

View larger version (25K): [in a new window]   Fig. 1. Setup of the light equipment used in this study to produce full-spectrum and monochromatic light. We used two xenon arc lamps, each lamp distributing light to three orientation cages. The light first passed through a column of double-distilled water and then entered a trifurcated glass fibre bundle, at which end a filter holder contained interference filters and/or neutral density filters to produce monochromatic light and/or to reduce light intensity, respectively. Before entering the orientation cage, the filtered light passed through 1-4 opaque Plexiglas sheets that acted as diffusers and that were placed directly on the top of the orientation cage so that the birds could not see the light source as a point.

View larger version (28K): [in a new window]   Fig. 2. Spectrometric measurements of the full-spectrum and monochromatic light used in the experiments, and the spectrum of an incandescent light bulb and the natural spectrum at sunset for comparison. The black line indicates the spectrum of the full-spectrum light. The dark green (560.5 nm), yellow-green (567.5 nm) and red (617 nm) lines indicate the spectrum of the monochromatic light as seen by the birds sitting under one opaque Plexiglas sheet. The composition of the measured full-spectrum light passing through neutral density filters of different transmissions and multiple layers of opaque Plexiglas sheets does not deviate substantially from the spectrum shown in black. For comparison, the orange line shows the spectrum of an incandescent light bulb, and the violet line shows the natural spectrum 14 min after sunset (reproduced from McFarland and Munz, 1975; 300-700 nm only). All spectra are standardised by taking the maximum value within the measured wavelength interval as 10.

View larger version (36K): [in a new window]   Fig. 3. Orientation behaviour of European robins tested under monochromatic light of different wavelengths and intensities (see Table 1 for details of wavelengths and intensities). Each triangle represents the heading of an individual bird. Filled triangles refer to unimodally directed individuals, and two open triangles denote axially distributed individuals (bisected triangles indicate the direction taken into account in the statistical analyses; unmarked, open triangles represent the other end of the axis). The arrows represent the mean orientation of the group and are drawn relative to the mean vector length (r). The inner broken circle gives the 5% significance level, and the outer broken circle gives the 1% significance level according to the Rayleigh test (Batschelet, 1981). See Table 2 for detailed statistical information. G, green light; GY, green-yellow light; R, red light; mN, magnetic north.

View larger version (27K): [in a new window]   Fig. 4. Orientation behaviour of European robins tested under full-spectrum light of different intensities in autumn. See Table 1 for details on intensities (W1-W20), Table 2 for detailed statistical information and Fig. 3 for further explanation of the graphs. Abbreviations: ac, after colour experiments; bc, before colour experiments.