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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.


Related in: MedlinePlus

Blackcaps can orient at 5° inclination but become random at 0° inclination.The orientation of the same group of birds in (a) an Earth strength magnetic field pointing towards geomagnetic North with an inclination of 5°, (b) an Earth strength magnetic field pointing towards geomagnetic North with an inclination of 0°, (c) an Earth strength magnetic field pointing towards −120° with an inclination of 5°, and (d) an Earth strength magnetic field pointing towards −120° with an inclination of 0°. The magnetic field intensity remained constant in all four conditions and all experiments were done in spring 2014. Each dot at the circle periphery represents the mean orientation angle of an individual bird. The arrows show the group mean directions and vector lengths, the dashed circles indicate the lengths of the group mean vectors needed for significance at the 0.05, 0.01, and 0.001 levels according to the Rayleigh-test, and the straight lines next to the group mean vectors show the 95% confidence interval limits for the group mean headings. mN = magnetic North; gN = geographic North.
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f2: Blackcaps can orient at 5° inclination but become random at 0° inclination.The orientation of the same group of birds in (a) an Earth strength magnetic field pointing towards geomagnetic North with an inclination of 5°, (b) an Earth strength magnetic field pointing towards geomagnetic North with an inclination of 0°, (c) an Earth strength magnetic field pointing towards −120° with an inclination of 5°, and (d) an Earth strength magnetic field pointing towards −120° with an inclination of 0°. The magnetic field intensity remained constant in all four conditions and all experiments were done in spring 2014. Each dot at the circle periphery represents the mean orientation angle of an individual bird. The arrows show the group mean directions and vector lengths, the dashed circles indicate the lengths of the group mean vectors needed for significance at the 0.05, 0.01, and 0.001 levels according to the Rayleigh-test, and the straight lines next to the group mean vectors show the 95% confidence interval limits for the group mean headings. mN = magnetic North; gN = geographic North.

Mentions: In spring 2014, the blackcaps oriented significantly in their appropriate migratory direction when they were tested in a magnetic field pointing towards magnetic north with a magnetic inclination of 5° (5°NMF: group mean orientation = 47° ± 37° [95% confidence interval], r [group mean’s vector length] = 0.61, N = 11, p < 0.05 [Rayleigh-test], Fig. 2a). When the horizontal component of the 5° inclined field was rotated −120°, the birds turned their orientation accordingly (5°CMF: group mean orientation = 294° ± 57°, r = 0.57, p < 0.05, N = 11, Fig. 2c). The Mardia-Watson-Wheeler test showed a significant difference in the orientation direction between 5°NMF and 5 °CMF (W = 6.402, df = 2, p = 0.041). In contrast, the birds were randomly oriented in both the north-pointing and the −120° rotated field when the inclination angle was set to 0° (spring 2014 0°NMF: group mean orientation = 16°, r = 0.26, p = 0.486 (ns), N = 11, Fig. 2b, 0 °CMF: group mean vector = 360°, r = 0.36, p = 0.214 (ns), N = 12, Fig. 2d). However, the distribution in the 0° inclination seemed to be not truly random, but bimodal in the 0°NMF condition (doubled angles: group mean vector = 120°/300°, r = 0.65, p < 0.01, N = 11), whereas there were no signs of bimodal orientation in the 0°CMF condition (group mean vector = 127°/307°, r = 0.31, p = 0.347 (ns), N = 12). The possible bimodal orientation in the 0°NMF condition is not oriented along the expected NE-SW axis. Therefore, we decided to test the birds again in both conditions in the following spring.


Migratory blackcaps can use their magnetic compass at 5 degrees inclination, but are completely random at 0 degrees inclination
Blackcaps can orient at 5° inclination but become random at 0° inclination.The orientation of the same group of birds in (a) an Earth strength magnetic field pointing towards geomagnetic North with an inclination of 5°, (b) an Earth strength magnetic field pointing towards geomagnetic North with an inclination of 0°, (c) an Earth strength magnetic field pointing towards −120° with an inclination of 5°, and (d) an Earth strength magnetic field pointing towards −120° with an inclination of 0°. The magnetic field intensity remained constant in all four conditions and all experiments were done in spring 2014. Each dot at the circle periphery represents the mean orientation angle of an individual bird. The arrows show the group mean directions and vector lengths, the dashed circles indicate the lengths of the group mean vectors needed for significance at the 0.05, 0.01, and 0.001 levels according to the Rayleigh-test, and the straight lines next to the group mean vectors show the 95% confidence interval limits for the group mean headings. mN = magnetic North; gN = geographic North.
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f2: Blackcaps can orient at 5° inclination but become random at 0° inclination.The orientation of the same group of birds in (a) an Earth strength magnetic field pointing towards geomagnetic North with an inclination of 5°, (b) an Earth strength magnetic field pointing towards geomagnetic North with an inclination of 0°, (c) an Earth strength magnetic field pointing towards −120° with an inclination of 5°, and (d) an Earth strength magnetic field pointing towards −120° with an inclination of 0°. The magnetic field intensity remained constant in all four conditions and all experiments were done in spring 2014. Each dot at the circle periphery represents the mean orientation angle of an individual bird. The arrows show the group mean directions and vector lengths, the dashed circles indicate the lengths of the group mean vectors needed for significance at the 0.05, 0.01, and 0.001 levels according to the Rayleigh-test, and the straight lines next to the group mean vectors show the 95% confidence interval limits for the group mean headings. mN = magnetic North; gN = geographic North.
Mentions: In spring 2014, the blackcaps oriented significantly in their appropriate migratory direction when they were tested in a magnetic field pointing towards magnetic north with a magnetic inclination of 5° (5°NMF: group mean orientation = 47° ± 37° [95% confidence interval], r [group mean’s vector length] = 0.61, N = 11, p < 0.05 [Rayleigh-test], Fig. 2a). When the horizontal component of the 5° inclined field was rotated −120°, the birds turned their orientation accordingly (5°CMF: group mean orientation = 294° ± 57°, r = 0.57, p < 0.05, N = 11, Fig. 2c). The Mardia-Watson-Wheeler test showed a significant difference in the orientation direction between 5°NMF and 5 °CMF (W = 6.402, df = 2, p = 0.041). In contrast, the birds were randomly oriented in both the north-pointing and the −120° rotated field when the inclination angle was set to 0° (spring 2014 0°NMF: group mean orientation = 16°, r = 0.26, p = 0.486 (ns), N = 11, Fig. 2b, 0 °CMF: group mean vector = 360°, r = 0.36, p = 0.214 (ns), N = 12, Fig. 2d). However, the distribution in the 0° inclination seemed to be not truly random, but bimodal in the 0°NMF condition (doubled angles: group mean vector = 120°/300°, r = 0.65, p < 0.01, N = 11), whereas there were no signs of bimodal orientation in the 0°CMF condition (group mean vector = 127°/307°, r = 0.31, p = 0.347 (ns), N = 12). The possible bimodal orientation in the 0°NMF condition is not oriented along the expected NE-SW axis. Therefore, we decided to test the birds again in both conditions in the following spring.

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.


Related in: MedlinePlus