Figure 1: Fall migrant monarch butterflies can use a magnetic field to orient in the proper southerly fall migratory direction.

(a) Flight simulator and coil system used in magnetic compass trials. (b) Irradiance curve of lighting conditions during flight simulator trials. Light measurement was taken inside the flight simulator, with the flight encoder and diffuser in position, at the position of the head of the tethered monarch butterfly during trials. (c) Orientation of individual directional fall monarchs during the fall 2012 (n=18) and 2013 (n=27) migration seasons (red dots; both years pooled: n=45) in a generated field with a 45° inclination angle, at a 141 μT field intensity. For the circle diagram, arrow indicates mean group orientation, shaded area is 95% confidence interval, mN is magnetic north.