Migratory blackcaps tested in Emlen funnels can orient at 85 but not at 88 degrees magnetic inclination

Abstract

Migratory birds are known to use the Earth's magnetic field as an orientation cue on their tremendous journeys between their breeding and overwintering grounds. The magnetic compass of migratory birds relies on the magnetic field's inclination, i.e. the angle between the magnetic field lines and the Earth's surface. As a consequence, vertical or horizontal field lines corresponding to 0 or 90 deg inclination should offer no utilizable information on where to find North or South. So far, very little is known about how small the deviations from horizontal or vertical inclination are that migratory birds can detect and use as a reference for their magnetic compass. Here, we asked: what is the steepest inclination angle at which a migratory bird, the Eurasian blackcap (Sylvia atricapilla), can still perform magnetic compass orientation in Emlen funnels? Our results show that blackcaps are able to orient in an Earth's strength magnetic field with inclination angles of 67 and 85 deg, but fail to orient in a field with 88 deg inclination. This suggests that the steepest inclination angle enabling magnetic compass orientation in migratory blackcaps tested in Emlen funnels lies between 85 and 88 deg. KEY WORDS: Bird navigation, Magnetic inclination compass, Functional range, Magnetoreception, Radical-pair mechanism, Sylvia atricapilla INTRODUCTIONFor 50 years, it has been known that birds are able to use the Earth's magnetic field for orientation (Merkel and Wiltschko, 1965). In contrast to a man-made compass that works on the basis of the polarity of the magnetic field, the bird's magnetic compass is an inclination compass (Wiltschko and Wiltschko, 1972;Wiltschko and Wiltschko, 1995). This means that birds do not differentiate between North and South but between poleward and equatorward (the direction in which the field lines and the Earth's surface form the smaller angle is defined as equatorward) (Wiltschko and Wiltschko, 1972;Wiltschko and Wiltschko, 1995). Therefore, at the magnetic equator or at the magnetic poles, where the inclination is 0 and 90 deg, respectively, the birds are faced with the problem that the Earth's magnetic field provides no or ambiguous directional information. In other words, there is no larger or smaller inclination angle to detect. Wiltschko and Wiltschko (Wiltschko and Wiltschko, 1992) suggested that for transequatorial migrants, the crossing of the equator serves as a trigger and changes the heading from equatorward to poleward. Cochran et al. (Cochran et al., 2004) suggested that the magnetic compass can be calibrated by celestial RESEARCH ARTICLEAG Neurosensorik/Animal Navigation, Institute of Biological and Environmental Sciences, University Oldenburg, D-26111 Oldenburg, Germany. *These authors contributed equally to this work ‡ Author for correspondence (nele.lefeldt@uni-oldenburg.de) Received 2 May 2014; Accepted 12 November 2014 cues and that this mechanism might help birds to cross the magnetic equator. Even with these suggestions, an interesting question remains: how wide...

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