This species is the most commonly recorded migrant butterfly visiting these shores every year but it is also the most easily recognisable which does influence the recordings. Whether it is actually the most numerous species to cross The English Channel is difficult to know because many other common UK resident species have their numbers swollen by butterflies arriving from the continent. Even our common ‘white’ butterflies are joined at times by many thousands of overseas insects.
Annual numbers of migrating butterflies vary wildly according to conditions where the butterflies are resident abroad and the weather systems which assist them on their journey to this country. There are some years when numbers of normally scarcer migrants such as Clouded Yellows and Painted Ladies far exceed those of the Red Admiral.
Perhaps the answer is that the Red Admiral is the most regularly reported butterfly migrating annually to the UK that is not considered to have a resident population here. But even this answer is not entirely satisfactory as there is increasing evidence to suggest that in some parts of the south the Red Admiral is managing to over-winter and if this trend continues then this will make future estimations of its migatory status also more difficult to assess.