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

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.


Blackcaps fail to orient in a 0° inclination magnetic field.In the spring migratory season 2015, the blackcaps were well oriented in their natural spring migratory direction in the geomagnetic field of Oldenburg (a) and turned their orientation accordingly when the magnetic field was turned 120° counter-clockwise (c). In contrast, the birds were randomly oriented when the inclination angle was set to 0° (0°NMF (b) and 0°CMF (d). For a description of the circular diagrams, see legend to Fig. 2.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
getmorefigures.php?uid=PMC5036058&req=5

f4: Blackcaps fail to orient in a 0° inclination magnetic field.In the spring migratory season 2015, the blackcaps were well oriented in their natural spring migratory direction in the geomagnetic field of Oldenburg (a) and turned their orientation accordingly when the magnetic field was turned 120° counter-clockwise (c). In contrast, the birds were randomly oriented when the inclination angle was set to 0° (0°NMF (b) and 0°CMF (d). For a description of the circular diagrams, see legend to Fig. 2.

Mentions: In spring 2015, birds were tested in the control conditions (67°NMF and 67 °CMF) and in the 0° inclination conditions (0°NMF and 0°CMF) to investigate further, whether the blackcaps were bimodal or randomly oriented in the field with a flat inclination. The birds oriented significantly in their appropriate migratory direction when they were tested in a magnetic field pointing towards magnetic north with a magnetic inclination of 67° (spring 2015 67°NMF: group mean orientation = 38° ± 28°, r = 0.66, p < 0.01, N = 13, Fig. 4a). When the horizontal component of the 67° inclined field was rotated −120°, the birds turned their orientation accordingly (spring 2015 67°CMF: group mean orientation = 284 ± 22°, r = 0.79, p < 0.001, N = 13, Fig. 4c). The Mardia-Watson-Wheeler test showed a significant difference in the orientation direction between the 67°NMF and the 67°CMF conditions (W = 19.965, df = 2, p < 0.001).


Migratory blackcaps can use their magnetic compass at 5 degrees inclination, but are completely random at 0 degrees inclination
Blackcaps fail to orient in a 0° inclination magnetic field.In the spring migratory season 2015, the blackcaps were well oriented in their natural spring migratory direction in the geomagnetic field of Oldenburg (a) and turned their orientation accordingly when the magnetic field was turned 120° counter-clockwise (c). In contrast, the birds were randomly oriented when the inclination angle was set to 0° (0°NMF (b) and 0°CMF (d). For a description of the circular diagrams, see legend to Fig. 2.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

f4: Blackcaps fail to orient in a 0° inclination magnetic field.In the spring migratory season 2015, the blackcaps were well oriented in their natural spring migratory direction in the geomagnetic field of Oldenburg (a) and turned their orientation accordingly when the magnetic field was turned 120° counter-clockwise (c). In contrast, the birds were randomly oriented when the inclination angle was set to 0° (0°NMF (b) and 0°CMF (d). For a description of the circular diagrams, see legend to Fig. 2.
Mentions: In spring 2015, birds were tested in the control conditions (67°NMF and 67 °CMF) and in the 0° inclination conditions (0°NMF and 0°CMF) to investigate further, whether the blackcaps were bimodal or randomly oriented in the field with a flat inclination. The birds oriented significantly in their appropriate migratory direction when they were tested in a magnetic field pointing towards magnetic north with a magnetic inclination of 67° (spring 2015 67°NMF: group mean orientation = 38° ± 28°, r = 0.66, p < 0.01, N = 13, Fig. 4a). When the horizontal component of the 67° inclined field was rotated −120°, the birds turned their orientation accordingly (spring 2015 67°CMF: group mean orientation = 284 ± 22°, r = 0.79, p < 0.001, N = 13, Fig. 4c). The Mardia-Watson-Wheeler test showed a significant difference in the orientation direction between the 67°NMF and the 67°CMF conditions (W = 19.965, df = 2, p < 0.001).

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&rsquo;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&deg; 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&deg; and 0&deg; inclination. At 5&deg; inclination, the birds oriented as well as they did in the normal 67&deg; 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&deg; and that these birds indeed seem completely unable to use their magnetic compass for orientation in a horizontal magnetic field.

No MeSH data available.