Direction of Spring Migration of Vanessa cardui (Nymphalidae) In Colorado

Abstract

Spring migration of 3016 Vanessa cardui (L.) migrants was studied in Colorado in 1992 and 1983, using standard vector methods. Migration was to the east-northeast/northeast (averaging 31° in 1992, 51° in 1983, east being 0° and north 90°), and this direction did not change significantly during the day, disproving a theory that migrants maintain a constant angle to the sun. The efficiency (unidirectionality) of migration is about 80% during peak migration, but drops to near zero afterward. Some spring adults in central Colo. overwintered there. Migrants mate-locate all day long, versus late in the day for non- migrants.