Migratory blackcaps can use their magnetic compass at 5 degrees inclination, but are completely random at 0 degrees inclination

Abstract

It is known that night-migratory songbirds use a magnetic compass measuring the magnetic inclination angle, i.e. the angle between the Earth’s surface and the magnetic field lines, but how do such birds orient at the magnetic equator? A previous study reported that birds are completely randomly oriented in a horizontal north-south magnetic field with 0° inclination angle. This seems counter-intuitive, because birds using an inclination compass should be able to separate the north-south axis from the east-west axis, so that bimodal orientation might be expected in a horizontal field. Furthermore, little is known about how shallow inclination angles migratory birds can still use for orientation. In this study, we tested the magnetic compass orientation of night-migratory Eurasian blackcaps (Sylvia atricapilla) in magnetic fields with 5° and 0° inclination. At 5° inclination, the birds oriented as well as they did in the normal 67° inclined field in Oldenburg. In contrast, they were completely randomly oriented in the horizontal field, showing no sign of bimodality. Our results indicate that the inclination limit for the magnetic compass of the blackcap is below 5° and that these birds indeed seem completely unable to use their magnetic compass for orientation in a horizontal magnetic field.

A light-dependent magnetoreception mechanism insensitive to light intensity and polarization

“…2) in equation (2.5). It has been argued that this feature, which arises from an avoided crossing of the energy levels of the electron-nuclear spin system of the radical pair [47], could afford a precise compass bearing that would be consistent with the ability of migratory songbirds to detect the axis of the magnetic field with an accuracy better than 58 [54][55][56] The magnetic signals (equation (2.5)) from cells A and B will only be identical when sin 2 j A ¼ sin 2 j B . Writing the two magnetic symmetry axes (related, as above, by a 1808 z-rotation) as the unit vectors z mA ¼ ðm x , m y , m z Þ and z mB ¼ ðÀm x , Àm y , m z Þ, and the magnetic field axis as the unit vector b ¼ ðb x , b y , b z Þ, we have:…”

Lidocaine is a nocebo treatment for trigeminally mediated magnetic orientation in birds

“…a 'map') information [1][2][3][4][5]. A large body of evidence suggests that birds possess a magnetic inclination 'compass' [1,2,6], which is embedded in their visual system [7][8][9][10]. The underlying magnetic compass sensor is hypothesized to be cryptochrome based [11][12][13][14], located in both of the birds' eyes [15,16], and the biophysical mechanism most likely relies on radical-pair-based spin-chemical reactions (for a comprehensive review, see [9]).…”

A hierarchy of compass systems in migratory birds

“…When the stars become visible, the birds recalibrate the star compass with respect to the previously recalibrated magnetic compass. However, such star compass recalibration only takes place if stars and their constellations are available during the cue-conflict treatment (Sjöberg and Muheim, 2016). This assumption, as the authors suggest, can explain why their garden warblers, pied flycatchers in the study by Guinchi et al (2015) and northern wheatears by Schmaljohann et al (2013) did not show recalibration of any compasses.…”

“…A recent study by Sjöberg and Muheim (2016) in a long-distance songbird migrant, the garden warbler, showed that orientation of free-flying warblers was not affected by pre-exposure to + 90° deflected magnetic field for 1 hour around sunset, and this species did not calibrate its compass systems either (Table 1C 24). Trying to explain the results of experiments in this study and all previous contradictory data obtained with other migratory birds, they suggested a new hypothesis, an 'extended unified theory' .…”

“…A review of this phenomenon was published in 2012 (Liu and Chernetsov, 2012). Since then, several new studies have been published in which researchers performed cue-conflict experiments and obtained results that did not support the hypothesis suggesting a crucial role of the sky view near the horizon proposed by Muheim et al (2006a) and later updated in 'extended unified theory' by Sjöberg and Muheim (2016). In the present review, we tried to understand why some migratory species calibrate their magnetic compass on sunset cues, whereas…”