To be fair bird movements are quite complicated: e.g. a Robin in the UK between September to March might be a member of a migratory or a non-migratory populatio

To be fair bird movements are quite complicated: e.g. a Robin in the UK between September to March might be a member of a migratory or a non-migratory population. And Blackcaps are an example of a mostly migratory species in the UK but where less migratory individuals come to feeding stations in winter months.