Fig. 2

Simulated migration routes based on different magnetic compass mechanisms. a Fixed (menotactic) magnetic compass (red) and b magnetoclinic compass orientation (orange). The routes are calculated in daily steps of 200 km, determining a new course for each step based on geomagnetic conditions at each daily departure location and assuming a constant geographic course within a step. Autumn migration routes were simulated with 1 Sept 2010 as initial departure date, spring migration routes with 1 April 2011 as departure date. For each compass mechanism, routes were centred on the Palaearctic-African (left panels) and Nearctic-Neotropic (right panels) migration systems, respectively. Dotted sections of magnetoclinic compass routes indicate situations where the angle of inclination of the Earth’s magnetic field was larger than the apparent angle of inclination, thus where a magnetoclinic compass could not be used, and the birds instead were assumed to orient a fixed magnetic compass instead. Great circle routes (dark grey dashed) are given for comparison to indicate the shortest routes. All maps are in Mercator projection. See Fig. 1 and text for more details