Butterfly-hostplant fidelity, vagrancy and measuring mobility from distribution maps

Vagrancy among butterflies, defined as individuals found in mapping units ( cells, squares) lacking larval hostplants, offers potential for measuring m obility among species. Herein, we investigate several simple measures of mo bility (vagrancy indices) based on the occurrence of vagrants in two indepe ndent data sets within the same region. We find that the vagrancy indices d o not simply reflect individual abundance, suggesting an innate component t o mobility. The vagrancy indices agree closely for the two data sets. They also correlate highly with a migration index, based on distances over which species have been recorded moving during the last 50 yr, indicating that f requency of movement corresponds closely with migration distances of specie s. The frequency of movement in some species, as measured by the vagrancy i ndices, is shown to be under-recorded, particularly nymphalids. In other sp ecies (e.g. Celastrina argiolus, Anthocharis cardamines, Pyronia tithonus) according to vagrancy indices, migration distances seem to have been previo usly under-recorded. We suggest how data collection can be improved for obt aining measurement of butterfly mobility.