Microsoft Word - 32778369-file00

Migratory birds are known to use the Earth’s magnetic field as an orientation cue on their 13 tremendous journeys between their breeding and overwintering grounds. The magnetic 14 compass of migratory birds relies on the magnetic field’s inclination, i.e. the angle between 15 the magnetic field lines and the Earth’s surface. As a consequence, vertical or horizontal field 16 lines corresponding to 0° or 90° inclination should offer no utilizable information on where to 17 find North or South. So far, very little is known about how small deviations from horizontal 18 or vertical inclination migratory birds can detect and use as a reference for their magnetic 19 compass. Here we ask: what is the steepest inclination angle at which a migratory bird, the 20 Eurasian blackcap (Sylvia atricapilla), can still perform magnetic compass orientation in 21 Emlen funnels? Our results show that blackcaps are able to orient in an Earth’s strength 22 magnetic field with inclination angles of 67° and 85°, but fail to orient in a field with 88° 23 inclination. This suggests that the steepest inclination angle enabling magnetic compass 24 orientation in migratory blackcaps tested in Emlen funnels lies between 85 and 88 degrees. 25