Migratory blackcaps can use their magnetic compass at 5 degrees inclination, but are completely random at 0 degrees inclination - PubMed (original) (raw)

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

Susanne Schwarze et al. Sci Rep. 2016.

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.

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Figures

Figure 1

Figure 1. 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. and Solov’yov et al. 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°.

Figure 2

Figure 2. 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.

Figure 3

Figure 3. Blackcaps are well oriented at 5° inclination in autumn.

(a) Unchanged geomagnetic field condition (magnetic inclination angle 67° of Oldenburg); (b) inclination set to 5° (5°NMF); (c) inclination 5°, horizontal component −120° (5°CMF). For a description of the circular diagrams, see legend to Fig. 2.

Figure 4

Figure 4. 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.

Figure 5

Figure 5. 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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