Figure 3: The inclination magnetic compass uses ultraviolet-A/blue light.

(a) Irradiance curves of lighting conditions during flight simulator trials. Black line indicates full spectrum light, blue line denotes filtered light that provided wavelengths of light >420 nm only, and purple line designates filtered light where only wavelengths of light >380 nm were available. (b) Migrants that oriented equatorwards during their first flight when tested under full spectrum lighting conditions (left; red; n=5) had significantly less directional flight, that is, Z<500 (right; blue; n=5), when tested under light that contained wavelengths >420 nm. (c) In contrast, monarchs that oriented equatorwards under full spectrum light during their initial flight (left; red; n=5), maintained similar oriented flight that was equatorwards when tested under lighting conditions that included wavelengths >380 nm (right; purple; n=5). For all trials, the inclination angle was 45° and the field intensity was 141 μT. In both b and c, bar graphs indicate the mean Z±s.e.m. score of animals for each group. Circular diagrams consist of the group orientation behaviour for each group in which each monarch plotted (dot) had a Z score >500. In each circular diagram, arrow indicates mean group orientation, shaded area is 95% confidence interval, and mN is magnetic north.