Ecology of red admiral migration

Abstract: The red admiral (Vanessa atalanta) is a migratory butterfly that can be found in North America, North Africa, Europe and some parts of Asia. In Europe the main part of the population spend winter in the Mediterranean region and the following spring leave this area to breed further to the north. At least one new generation is then produced in the summer region before they return to the south again in autumn. Among the European butterfly species that are known to migrate, the red admiral is the perhaps most regular in appearance making it a good candidate for studies of butterfly migration. In my thesis, I have studied the migration ecology of this species. Using AFLP analysis I found two distinct sub-populations with a predominantly western and eastern distribution, but in northern Europe there were large variation between sites considering which group were dominating. Individuals with intermediate genotype were rare, suggesting isolatory mechanisms being present. The winter ecology of red admirals was studied in Italy and when comparing the lipid content of the autumn and spring samples I found no evidence of hibernation during winter. I did find a difference in wing length between samples from the two seasons suggesting that reproduction takes place during winter. To be able to delineate the natal region of captured butterfly migrants I evaluated the geographical resolution of stable isotope methods in Sweden using the peacock (Inachis io), a species closely related to the red admiral. Isotopes of stable hydrogen is probably the most useful indicator for finding the origin of individuals of migratory butterflies in Europe even though variations due to difference in the local amount of precipitation can have a large impact on the results. Deuterium ratios in red admirals captured in Italy during spring indicated that they to some extent originate from North Africa. Samples from Spain and France appears to be isolated from Scandinavia, Italy and eastern Europe and samples from coastal regions showed larger variation. I studied orientation in red admirals on autumn migration using circular flight cages and video imaging. The main flight direction in early autumn was towards the north in 1999 but towards the south in 2002 and 2005. I also found evidence for a time-compensated sun compass but only during one year. Combining visual counts of red admirals at Falsterbo in southwestern Sweden with local weather data I have shown that red admirals selectively choose to migrate on days with easterly winds of low speed and clear skies. It was also evident that the date of peak migration as well as absolute numbers observed over the season is highly variable between years.