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Aim  To relate variation in the migration capacity and colonization ability of island communities to island geography and species island occupancy. Location  Islands off mainland Britain and Ireland. Methods  Mean migration (transfer) capacity and colonization (establishment) ability (ecological indices), indexed from 12 ecological variables for 56 butterfly species living on 103 islands, were related to species nestedness, island and mainland source geography and indices using linear regression models, RLQ analysis and fourth‐corner analysis. Random creation of faunas from source species, rank correlation and rank regression were used to examine differences between island and source ecological indices, and relationships to island geography. Results  Island butterfly faunas are highly nested. The two ecological indices related closely to island occupancy, nestedness rank of species, island richness and geography. The key variables related to migration capacity were island area and isolation; for colonization ability they were area, isolation and longitude. Compared with colonization ability, migration capacity was found to correlate more strongly with island species occupancy and species richness. For island faunas, the means for both ecological indices decreased, and variation increased, with increasing island species richness. Mean colonization ability and migration capacity values were significantly higher for island faunas than for mainland source faunas, but these differences decreased with island latitude. Main conclusions  The nested pattern of butterfly species on islands off mainland Britain and Ireland relates strongly to colonization ability but especially to migration capacity. Differences in colonization ability among species are most obvious for large, topographically varied islands. Generalists with abundant multiple resources and greater migration capacity are found on all islands, whereas specialists are restricted to large islands with varied and long‐lived biotopes, and islands close to shore. The inference is that source–sink dynamics dominate butterfly distributions on British and Irish islands; species are capable of dispersing to new areas, but, with the exception of large and northern islands, facilities (resources) for permanent colonization are limited. The pattern of colonization ability and migration capacity is likely to be repeated for mainland areas, where such indices should provide useful independent measures for assessing the conservation status of faunas within spatial units.

Summary 1.Predictions on species richness and incidence of species are made using data for three scales of mapping from the Greater Manchester butterfly atlas; for the whole of the conurbation (2 km×2 km scale) and for two sample areas centred on the Mersey Valley (1 km×1 km scale; 1 ha scale). Predictions are based on data for recording effort, altitude, biotopes, host‐plants and nectar resources. 2.Data for Greater Manchester indicate that substantial shortfalls may occur in recording butterfly species for atlases despite the fact that butterflies are generally easily identified and well supported with recorders. Shortfalls tend to be larger for species with fewer records, indicating that some species may be more easily overlooked than others. 3.The results demonstrate that targeting squares for re‐survey is necessary and feasible. The predictions have other valuable research applications, the most important of which is being able to assess the accuracy of distribution maps, to correct them, and to make projections of distribution changes. 4.Predictions may be enhanced by improvements to mapping in three ways: (i) Collecting data on recording effort. Variation in recording effort typically accounts for differences in species richness and incidence of species more than any other variable; (ii) Collecting data on biotopes and specific resources. The present results are promising and demonstrate that the collection of environmental data linked to a suitable sampling frame could facilitate knowledge of the distribution of species over extensive areas that remain under‐recorded; and (iii) Distinguishing between breeding individuals and vagrants. Vagrancy is a problem associated both with species and scale. Although species vary substantially in their capacity to migrate beyond their habitats, the effect of vagrancy on distribution maps becomes an increasingly large problem as the grain of mapping (size of recording units) decreases. It is suggested that over‐recording can be a problem, particularly when mapping is fine‐grained.

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Vagrancy among butterflies, defined as individuals found in mapping units (cells, squares) lacking larval hostplants, offers potential for measuring mobility among species. Herein, we investigate several simple measures of mobility (vagrancy indices) based on the occurrence of vagrants in two independent data sets within the same region. We find that the vagrancy indices do not simply reflect individual abundance, suggesting an innate component to 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 frequency of movement corresponds closely with migration distances of species. The frequency of movement in some species, as measured by the vagrancy indices, is shown to be under‐recorded, particularly nymphalids. In other species (e.g. Celastrina argiolus. Authocharis cardamines. Pyronia tithonus) according to vagrancy indices, migration distances seem to have been previously under‐recorded. We suggest how data collection can be improved for obtaining measurement of butterfly mobility.