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Bioelectromagnetism

When robins were tested in experimental felds with diferent intensities, it became evident that

their magnetic compass is narrowly tuned to the ambient magnetic feld (Wiltschko et al., 2011). At the

test site in Frankfurt am Main, Germany (50°08′N, 8°40′E), the local GMF has an intensity of ∼46 μT

(Wiltschko et al., 2011). Robins caught and kept at this intensity were disoriented when the total intensi­

ties were decreased or increased by about 30%, indicating a narrow functional window (Wiltschko and

Wiltschko, 1995, 2007). Te disorientation in higher felds was especially surprising, because it clearly

showed that the loss of orientation was not caused by the intensity getting below threshold (Wiltschko

et al., 2011). Further tests showed that the functional window is fexible and can be adjusted to intensities

outside the normal functional range (Wiltschko et al., 2011). Robins regained their ability to orient when

they are exposed to lower or higher intensities, with an exposure of about 1 hour at 92 μT sufcient to

enable them to orient at this intensity (Wiltschko et al., 2006a). At the same time, the birds did not lose

their ability to orient in the local GMF (Wiltschko et al., 2011). Tis adjustment to new intensities is nei­

ther a shif nor a simple enlargement of the functional range; rather, experiencing an intensity outside

the normal functional range seems to establish a new functional window around the respective intensity

(Wiltschko and Wiltschko, 1995, 2007).

Moreover, Wiltschko and Wiltschko (1972) speculated with great insight that on the whole, this mag­

netic compass represents a highly fexible direction-fnding system. Tey further estimated that its abil­

ity to adjust to a varying intensity range makes it independent of any secular variation in total intensity,

and the fact that it does not use the polarity of the magnetic felds so-called “polarity compass” means

that it is not afected by the geomagnetic reversals that have taken place several times since the phylo­

genetic origin of birds (Runcorn, 1969). Te published data of inclination compass in European robin

are shown by Wiltschko and Wiltschko (2005), compiled from Wiltschko and Wiltschko (1999) and

Wiltschko et al. (2001). Orientation behavior of migrating European robins in spring was tested in the

local GMF and in two experimental felds (Wiltschko and Wiltschko, 1999, 2005; Wiltschko et al., 2001).

Te triangles at the periphery of the circle mark mean headings of individual birds, the arrows represent

the grand mean vectors with their lengths proportional to the radius of the circle. Te two inner circles

are the 5% and the 1% signifcance border of the Rayleigh test.

Subsequent research has revealed when the birds are blindfolded, they cannot perceive the magnetic

feld, and therefore, the magnetic perception requires light radiation or stimulation (Wiltschko and

Wiltschko, 1999, 2001; Wiltschko et al., 2000a, b, 2001). More specially, this behavior was recorded under

565 nm green light at an intensity of 2.1 mW/m² as shown by Wiltschko and Wiltschko (2005), compiled

from Wiltschko et al. (2000b, 2001) and Wiltschko and Wiltschko (2002). Te orientation behavior of

European robins in spring was monitored under monochromatic lights of diferent wavelengths.

Wiltschko et al. (2006b) subjected migratory Australian silvereyes, Zosterops lateralis, to a short,

strong magnetic pulse and tested their subsequent response under diferent magnetic conditions. In

the local GMF, the birds preferred easterly headings as before, and when the horizontal component of

the magnetic feld was shifed 90° anticlockwise, they altered their headings accordingly northwards

(Wiltschko et al., 2006b). In a feld with the vertical component inverted, the birds reversed their head­

ings westwards, indicating that their directional orientation was controlled by the normal inclination

compass (Wiltschko et al., 2006b).

Tus, in addition to the inclination compass, another important characteristic of the avian magnetic

compass is its “light-dependency.” Wiltschko et al. (2011) presented wavelength dependency of the avian

magnetic compass (Figure 4.5, data from Wiltschko et al., 1993, 2007, 2010; Wiltschko and Wiltschko,

1998; Rappl et al., 2000; Muheim et al., 2002). Normal compass orientation requires light from the short-

wavelength part of the spectrum. European robins and Australian silvereyes are well oriented in their

migratory directions under 373 nm ultraviolet (UV), 424 nm blue, 502 nm turquoise, and 565 nm green

light. Under 590 nm yellow and beyond, they were disoriented, indicating that their magnetorecep­

tion system works no longer properly under longer wavelength (Wiltschko et al., 1993; Muheim et al.,

2002; Wiltschko and Wiltschko, 2007). Experiments using interference flters with a half-band width

of only 10 nm could narrow down the onset of disorientation in robins even further to between 561 and