Increased abundance of the red admiral butterfly Vanessa atalanta in Britain: the roles of immigration, overwintering and breeding within the country

The migratory butterfly Vanessa atalanta increased in abundance at monitore d sites in Britain from 1976 to 1996. Three possible causes of the increase are improved winter survival within Britain, greater breeding success with in Britain, and increased immigration. Trends during most of the season mere similar to those of immigrant or over wintered 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 se emed unlikely that overwintering in Britain was important. Thus the increas e in abundance was probably due to increased immigration. Incidental to the main study, the mean index per site per year was closely correlated with t he collated index, the usual measure of annual fluctuations. This agreement suggests that the mean index may be a useful check for trends in monitorin g data for other wide-ranging organisms.