Seasonally adaptive migratory headings mediated by a sun compass in the painted lady butterfly, Vanessa cardui

Many insects undertake long-distance migrations to exploit seasonally variable conditions at high latitudes, but the mechanisms used by migrants to select and maintain beneficial flight headings are poorly understood. Using computerized flight simulators, we performed controlled experiments to test the ability of an obligate migrant butterfly (Vanessa cardui) to orient in seasonally advantageous directions (i.e. northwards in spring and southwards in autumn). We also investigated the compass mechanism used to select and maintain these headings. Laboratory-reared autumn-generation butterflies flown in the U.K. displayed a highly significant mean orientation towards the south-southwest, consistent with return migration to winter breeding sites. However, seasonally adaptive flight headings were not observed in wild-caught adults flown at the same time. Spring-generation adults caught in Gibraltar (presumed to be migrating from winter breeding sites in North Africa into Europe) showed no evidence of northward flight headings, but produced a wide scatter of flight headings with a mean direction towards the west. Butterflies flown in the simulators when the sky was not visible produced a random scatter of flight headings and less-directed flight tracks, providing evidence that migrating V. cardui use a sun compass to select and maintain their flight headings. However, when butterflies were subjected to a 6 h clock shift, no change in orientation was observed relative to the control group. Field evidence for a return migration in autumn by V. cardui is surprisingly scarce in the literature, but we conclude that the species does attempt such southward movements and that individuals use a sun compass to select their migratory heading.