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THE RED ADMIRAL
This striking butterfly, Vanessa atalanta, is a popular visitor to gardens. Until recently little was understood about its behaviour and much still remains a mystery.

A robust butterfly, its wings are adapted for prolonged flight over land and sea. It is not a species which hibernates, but overwinters in France and Spain, whilst further south still in North Africa it can continue to breed throughout our winter months. In mild winters, Red Admirals can sometimes be found overwintering in the south and west of England. On cold days they rest, emerging on warm sunny days to feed and drink - unless they can do so at regular intervals they do not survive here.

In spring, migration northwards takes place steadily, those arriving in Britain (at any time between late March and late May) go on to breed and produce the butterflies that are such a welcome sight in August and September in our gardens. Curiously, little is known about where our breeding Red Admirals mate - until recently there was only one well documented sighting of a paired couple in Britain. Do the immigrant butterflies mate further south or only at night?

Recent mild winters have caused a spate of reports of Red Admirals over-wintering in Britain (there were 24 reports from 16 counties on 1st January 2007 alone), especially but not exclusively in the south, and quite a few sightings of mating pairs. These might not be typical matings, however, as the low temperatures at that time of year would inhibit flight and prevent them rising out of sight, perhaps into tree tops, as they normally would. In Cumbria I recorded my earliest ever Red Admiral on 1st March 2007, probably one that had over-wintered as there was no evidence of any other influx.

Eggs are laid on the common nettle. When hatched the larva constructs a 'tent' by drawing leaf edges together with silky threads. Four moults take place before pupation, with the chrysalis protected in another leaf tent. The adult butterfly emerges about two months (depending on prevailing temperatures) after egg laying.

During September and October the direction of migration reverses. What triggers this is not known - shorter days, lower temperatures or angle of sun are possible explanations. Now large numbers may build up along the south coast of England, where they feed up furiously for several days. Suddenly, usually when the wind veers to the north, they are gone in a day or so crossing the Channel overnight. Some stragglers, possibly because they have not been able to feed up enough, are left behind and attempt to overwinter here - usually without success. However, in recent years with very mild winters, there have even been amazing cases where larval development and pupation have been closely watched during the winter.

Why some Red Admirals fly far enough south to continue breeding but others overwinter in France and Spain without breeding is not known. Are there two genetic strains of Red Admiral in Europe?

The Red Admiral is a cosmopolitan species, being found in Central and North America. Here it seems to follow a similar breeding and migration pattern. It is a common species in New Zealand, but it is unclear whether it was introduced here or not. One wonders what the migration pattern is between the two islands - current evidence suggests it is largely sedentary?