Each autumn, large numbers of red admirals migrate throughout northern Europe,flying south, to reach areas with conditions suitable for surviving the winter. We observed the visible butterfly migration at Falsterbo peninsula, the southwesternmost point in Sweden, where red admirals are seen most autumns. ying towards the Danish coast on their way to more southern parts of Europe. Weather parameters from a local weather station were used to analyse what factors are important for red admiral migration across the sea. Wind direction was among the important weather variables affecting the initiation of the migratory departure; most other studies of butterfly migration reported no large effect of wind direction. This difference is probably because the butterflies in our study were about to cross open sea for more than 20 km, whereas most previous studies were from inland locations where butterflies could avoid wind effects by. ying close to the ground or on the lee side of topographical features. Other important weather variables affecting red admiral migration at Falsterbo were low wind speed and clear skies. The flight direction at Falsterbo was towards the west, which is in contrast to the southward direction generally reported during autumn migration in this species. This is probably because the red admirals followed both the local topography and the closest route to land on the other side and therefore deviated from the normally preferred direction to minimize flight over open water. (c) 2008 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.