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Quantum Compass of Migratory Birds

4.3 Magnetic Sense via Radical Pair Mechanism

Surprisingly, it has been reported that migratory birds seem to determine the direction of migration

using the GMF through the above-mentioned similar reaction process in their certain physiological

sensors. In brief, it has been reported that the GMF would afect the singlet-triplet (S-T) interconver­

sion in an orientation-dependent manner relative to the sensor molecule, leading to a change in the S-T

yield that would, in turn, trigger a physiological and behavioral response in migratory birds (Rodgers

and Hore, 2009; Hore and Mouritsen, 2016). We review and describe the history of several distinguished

studies on the RPM models below.

For the magnetic compass of migratory birds, the RPM model for quantum-assisted magnetic sensing

was frst proposed by Schulten et al. in 1978. It is suggested that this hardcore theoretical physics paper

formulated the RPM hypothesis of magnetoreception for the frst time, and it is now clear that it was

decades ahead of its time (Mouritsen, 2018). At the same time, since the 1970s, Mr. and Mrs. Wiltschko

have begun to investigate the magnetic sensation of a migratory bird, European robin (Erithacus rubec­

ula) in terms of ethology, which is the study of animal behavior (Wiltschko and Wiltschko, 1972). Later,

they also found the magnetic compass of non-migratory birds, such as homing pigeons Columba livia

(Wiltschko et al., 1981; Wiltschko and Wiltschko, 1998; Fleissner et al., 2003, 2007; Mora et al., 2004,

2014; Falkenberg et al., 2010; Wilzeck et al., 2010; Alexander et al., 2020; Rotov et al., 2020), and domestic

chickens Gallus gallus (Freire et al., 2005, 2008; Wiltschko et al., 2007; Denzau et al., 2013a, b).

By altering the magnetic feld, it has been experimentally demonstrated that it is possible to change

the direction of fight of birds within a cage. Wiltschko and Wiltschko (1972) discovered the so-called

“inclination compass” or “axial compass” in European robin. Wiltschko et al. (2011) presented a sche­

matic section through the GMF from the west to illustrate the functional mode of the inclination com­

pass (Figure 4.4). In principle it is possible to detect the inclination of the magnetic feld lines, but which

is north polarity or south polarity. It is impossible to obtain information on the direction itself. Te

inclination compass in the magnetic sense that only the compass information on the dip angle, which is

the orientation component of the magnetic vector, can be obtained. Tus, the avian magnetic compass

does not distinguish between magnetic “north” and “south” as indicated by polarity, but between “pole­

ward” where the feld lines point to the ground, and “equatorward” where they point upward (Wiltschko

and Wiltschko, 2005; Wiltschko et al., 2011).

FIGURE 4.4 Schematic section through the GMF from the west to illustrate the functional mode of the inclina­

tion compass (Wiltschko et al., 2011). N, S, North and South; He, vector of the GMF; H, vector of the experimental

feld; Hh, Hv, horizontal and vertical components of the magnetic felds; g, gravity vector. Te arrowheads indicate

the polarity of the felds, with mN, mS, indicating magnetic North and magnetic South, respectively. Te axial

direction of the vector and its inclination, i.e., its relation to gravity is crucial for the inclination compass, with p, e

indicating “poleward” and “equatorward,” the readings of the inclination compass. Te birds fy towards the direc­

tions that they assume to be their spring migratory direction. (Reproduced with permission from Wiltschko et al.,

2011, Copyright 2011, Elsevier.)