Understanding the migration ecology of European red admirals Vanessa atalanta using stable hydrogen isotopes

Tracking migratory movement of small animals with variable migration patterns is difficult with standard mark-recapture methods or genetic analysis. We used stable hydrogen isotope (delta D) measurements of wings from European red admirals Vanessa atalanta to study several aspects of this species' migration. In the central part of southern Europe we found large differences in delta D values between red admirals sampled in autumn and spring supporting the hypothesis that reproduction takes place in the Mediterranean region during winter. There was also an apparent influx to southern Europe in the spring of individuals with a more southerly origin, since many samples had higher delta D values and similar to those expected from coastal areas of North Africa. We found a clear seasonal difference in the delta D values of red admirals sampled in northern Europe. Spring migrants arriving in northern Europe generally had high delta D values that indicated a southerly origin. In autumn, delta D values suggested that red admirals were mostly from regions close to the sampling sites, but throughout the sampling period there were always individuals with delta D values suggesting non-local origins. The migration pattern of this species is supposedly highly variable and plastic. delta D differences between individuals in the western part of Europe were generally small making migratory patterns difficult to interpret. However, butterflies from western Europe were apparently isolated from those from north-eastern Europe, since delta D values in the western region rarely corresponded to those of autumn migrants from the north-east. Use of delta D data for inferring butterfly migration in Europe is complex, but our study showed that this technique can be used to help uncover previously unknown aspects of red admiral migration.