The migratory butterfly Vanessa atalanta increased in abundance at monitored sites in Britain from 1976 to 1996. Three possible causes of the increase are improved winter survival within Britain, greater breeding success within Britain, and increased immigration.
Trends during most of the season were similar to those of immigrant or overwintered individuals in spring; thus the evidence does not support greater breeding success in Britain. As abundance in spring was not correlated with abundance in the previous autumn, when trend was taken into account, it seemed unlikely that overwintering in Britain was important. Thus the increase in abundance was probably due to increased immigration. Incidental to the main study, the mean index per site per year was closely correlated with the collated index, the usual measure of annual fluctuations. This agreement suggests that the mean index may be a useful check for trends in monitoring data for other wide-ranging organisms.