Migration of the painted lady butterfly, Vanessa cardui, to north-eastern Spain is aided by African wind currents

1. Thousands of records of migratory butterfly species such as Vanessa cardui flying just above ground-level on fixed compass bearings have led to the common belief that these insects migrate within the so-called 'flight-boundary layer', where movements are relatively independent of the wind. 2.Given the selective advantages of windborne migration and the existence of a number of observations of flights of V. cardui from the upper levels of the atmosphere, we tested the hypothesis that migration from North Africa to southern Europe in this species is influenced by synoptic-scale wind currents. 3. Even with modern technology, it is extremely difficult to observe high-altitude flights directly, so we rely on an indirect approach that examines whether or not arrival peaks in north-eastern Spain are associated with winds blowing from Africa. 4. Arrivals of V. cardui were determined for the spring period (1 March-27 June, 1997-2006) at 79 sites in the Catalan Butterfly Monitoring Scheme. Wind patterns were described on the basis of synoptic-scale maps, transport models and back-trajectories calculated for each day of the spring period. 5. We found a strong association between migration and winds from North Africa, both for the whole data set (1997-2006; chi(2) = 4.7, P = 0.03) and for a restricted data set that excludes years in which the species was very scarce (chi(2) = 7.26, P = 0.007). 6. Episodes of massive northward migration within the species' flight-boundary layer also coincided with spells of winds from North Africa, suggesting a connection between low-altitude (observational) and high-altitude flights (inferred from wind patterns). 7. Finally, on the assumption that migration in V. cardui is windborne, a source-receptor transport model applied to spring abundance data in north-eastern Spain enables us to identify the most probable population source areas in North Africa.