For most of the twentieth century British recorders described Vanessa atalanta as a summer immigrant. Adults arrived from continental Europe and North Africa in spring and early summer, bred on nettles, and the resulting generation was assumed to die or emigrate before winter. Winter sightings were noted as anomalies.
That picture has shifted. Systematic recording schemes have documented increasing numbers of adults active in December, January and February in southern Britain and Ireland, and evidence of successful overwintering in sheltered sites. Some individuals remain in a state of reduced activity and fly on mild days. The species is now generally described as both a migrant and a partial resident in Britain, with the resident component expanding northwards.
The distinction between migration and residency in this species is unusually hard to draw. Individuals continue to move seasonally, and both southward autumn flights and northward spring arrivals are recorded, so a butterfly seen in February may be a survivor or an unusually early arrival. Separating these requires marking or isotope work that has not been widely applied.