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

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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.

No MeSH data available.


An illustration indicating how the Earth’s magnetic field at the magnetic equator may appear to a bird, which “sees” the magnetic field.Hypothetical signal modulation patterns using the assumptions also used in Ritz et al.26 and Solov’yov et al.32 for a bird changing its viewing direction clockwise in 90° increments in a magnetic field of 0° inclination. The four circles represent a full 360° sweep, showing all cardinal directions, from north (left circle) to west (right circle). Each “view” covers 180°.
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f1: An illustration indicating how the Earth’s magnetic field at the magnetic equator may appear to a bird, which “sees” the magnetic field.Hypothetical signal modulation patterns using the assumptions also used in Ritz et al.26 and Solov’yov et al.32 for a bird changing its viewing direction clockwise in 90° increments in a magnetic field of 0° inclination. The four circles represent a full 360° sweep, showing all cardinal directions, from north (left circle) to west (right circle). Each “view” covers 180°.

Mentions: Because the inclination angle is 0°, the birds cannot distinguish between “poleward” and “equatorward” at the magnetic equator by using their magnetic inclination compass. Wiltschko and Wiltschko9 and Wiltschko47 reported that birds using a magnetic inclination compass for orientation are randomly oriented in a magnetic field with 0° inclination. We found this result somewhat surprising, because even though the birds should not be able to discriminate north from south, they should be able to differentiate between the north-south axis and the east-west axis (see Fig. 1).


Migratory blackcaps can use their magnetic compass at 5 degrees inclination, but are completely random at 0 degrees inclination
An illustration indicating how the Earth’s magnetic field at the magnetic equator may appear to a bird, which “sees” the magnetic field.Hypothetical signal modulation patterns using the assumptions also used in Ritz et al.26 and Solov’yov et al.32 for a bird changing its viewing direction clockwise in 90° increments in a magnetic field of 0° inclination. The four circles represent a full 360° sweep, showing all cardinal directions, from north (left circle) to west (right circle). Each “view” covers 180°.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC5036058&req=5

f1: An illustration indicating how the Earth’s magnetic field at the magnetic equator may appear to a bird, which “sees” the magnetic field.Hypothetical signal modulation patterns using the assumptions also used in Ritz et al.26 and Solov’yov et al.32 for a bird changing its viewing direction clockwise in 90° increments in a magnetic field of 0° inclination. The four circles represent a full 360° sweep, showing all cardinal directions, from north (left circle) to west (right circle). Each “view” covers 180°.
Mentions: Because the inclination angle is 0°, the birds cannot distinguish between “poleward” and “equatorward” at the magnetic equator by using their magnetic inclination compass. Wiltschko and Wiltschko9 and Wiltschko47 reported that birds using a magnetic inclination compass for orientation are randomly oriented in a magnetic field with 0° inclination. We found this result somewhat surprising, because even though the birds should not be able to discriminate north from south, they should be able to differentiate between the north-south axis and the east-west axis (see Fig. 1).

View Article: PubMed Central - PubMed

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.

No MeSH data available.