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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.


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Pooled data from the 0° inclination conditions of spring 2014 and 2015.When the data obtained in the 0° inclination conditions from spring 2014 and spring 2015 are pooled and recalculated for each individual bird, so that each bird only contributes one mean direction seen over the two years in total, it becomes clear that we see no signs of any bimodal NE-SW orientation in the 0°NMF condition or of any E-W bimodal orientation in the 0°CMF condition as might have been expected, if the birds could determine the axis of the magnetic field. (a) Pooled data for 0°NMF and (b) pooled data for 0°CMF. For a description of the circular diagrams, see legend to Fig. 2.
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f5: Pooled data from the 0° inclination conditions of spring 2014 and 2015.When the data obtained in the 0° inclination conditions from spring 2014 and spring 2015 are pooled and recalculated for each individual bird, so that each bird only contributes one mean direction seen over the two years in total, it becomes clear that we see no signs of any bimodal NE-SW orientation in the 0°NMF condition or of any E-W bimodal orientation in the 0°CMF condition as might have been expected, if the birds could determine the axis of the magnetic field. (a) Pooled data for 0°NMF and (b) pooled data for 0°CMF. For a description of the circular diagrams, see legend to Fig. 2.

Mentions: When the birds were tested in the 0° inclination fields (0°NMF and 0°CMF), the birds could not use their magnetic compass anymore (spring 2015 0°NMF: group mean orientation = 8°, r = 0.2, p = 0.709 (ns), N = 9 Fig. 4b; 0°CMF: group mean orientation = 143°, r = 0.08, p > 0.9 (ns), N = 12, Fig. 4d). Furthermore, the orientation at 0° inclination revealed no bimodality of the orientation in the 0°NMF condition (doubled angles: group mean vector = 23°/203°, r = 0.11, p > 0.9 (ns), N = 9), or in the 0°CMF condition (group mean vector = 157°/337°, r = 0.32, p = 0.299 (ns), N = 12). The same holds when the data from the spring seasons 2014–2015 are pooled (pooled data 0°NMF: group mean orientation = 25°, r = 0.33, p = 0.140 (ns), N = 17, Fig. 5a; 0°CMF: group mean orientation = 4°, r = 0.08, p = 0.882 (ns), N = 20, Fig. 5b; double angles: 0°NMF: group mean vector = 115°/295°, r = 0.23, p = 0.446 (ns), N = 17; 0°CMF: group mean vector = 128°/308°, r = 0.20, p = 0.455 (ns), N = 20). If all 0°NMF and 0°CMF data from both seasons are corrected for magnetic north and pooled, also no bimodality was observed (double angles: relative magnetic North: group mean vector = 94°/274°, r = 0.20, p = 0.513 (ns), N = 17).


Migratory blackcaps can use their magnetic compass at 5 degrees inclination, but are completely random at 0 degrees inclination
Pooled data from the 0° inclination conditions of spring 2014 and 2015.When the data obtained in the 0° inclination conditions from spring 2014 and spring 2015 are pooled and recalculated for each individual bird, so that each bird only contributes one mean direction seen over the two years in total, it becomes clear that we see no signs of any bimodal NE-SW orientation in the 0°NMF condition or of any E-W bimodal orientation in the 0°CMF condition as might have been expected, if the birds could determine the axis of the magnetic field. (a) Pooled data for 0°NMF and (b) pooled data for 0°CMF. 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

f5: Pooled data from the 0° inclination conditions of spring 2014 and 2015.When the data obtained in the 0° inclination conditions from spring 2014 and spring 2015 are pooled and recalculated for each individual bird, so that each bird only contributes one mean direction seen over the two years in total, it becomes clear that we see no signs of any bimodal NE-SW orientation in the 0°NMF condition or of any E-W bimodal orientation in the 0°CMF condition as might have been expected, if the birds could determine the axis of the magnetic field. (a) Pooled data for 0°NMF and (b) pooled data for 0°CMF. For a description of the circular diagrams, see legend to Fig. 2.
Mentions: When the birds were tested in the 0° inclination fields (0°NMF and 0°CMF), the birds could not use their magnetic compass anymore (spring 2015 0°NMF: group mean orientation = 8°, r = 0.2, p = 0.709 (ns), N = 9 Fig. 4b; 0°CMF: group mean orientation = 143°, r = 0.08, p > 0.9 (ns), N = 12, Fig. 4d). Furthermore, the orientation at 0° inclination revealed no bimodality of the orientation in the 0°NMF condition (doubled angles: group mean vector = 23°/203°, r = 0.11, p > 0.9 (ns), N = 9), or in the 0°CMF condition (group mean vector = 157°/337°, r = 0.32, p = 0.299 (ns), N = 12). The same holds when the data from the spring seasons 2014–2015 are pooled (pooled data 0°NMF: group mean orientation = 25°, r = 0.33, p = 0.140 (ns), N = 17, Fig. 5a; 0°CMF: group mean orientation = 4°, r = 0.08, p = 0.882 (ns), N = 20, Fig. 5b; double angles: 0°NMF: group mean vector = 115°/295°, r = 0.23, p = 0.446 (ns), N = 17; 0°CMF: group mean vector = 128°/308°, r = 0.20, p = 0.455 (ns), N = 20). If all 0°NMF and 0°CMF data from both seasons are corrected for magnetic north and pooled, also no bimodality was observed (double angles: relative magnetic North: group mean vector = 94°/274°, r = 0.20, p = 0.513 (ns), N = 17).

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