The Myth of Red Admiral Hibernation

 

It has been a warm autumn so far in 2021, subsequently small numbers of butterflies are still lingering on the wing, with two Red Admirals recorded locally on Fordon Banks on the 2nd and one in York City on the 3rd. Certainly in the case of the two local bones the area they occupied on the southern edge of some scrub provided a temperature around 5 degrees warmer than shaded areas nearby. Indeed my house 10 minutes drive away was 9 degrees, whereas Fordon was a balmy 14 degrees. 

Red Admiral are a migratory species in the UK, which have a life cycle much like the Painted Lady. There has long been a myth amongst the ecological community in the UK that this species hibernates here in increasing numbers ready for the spring. Small numbers appear to hibernate here often in exposed situations on tree trunks, similarly to where they roost during the night at other times of year. However it is likely that this is all they are doing, these late emerging individuals struggle to muster the energy or conditions to migrate south late in the year so attempt to roost/hibernate. Most of which die off due to frosts killing them in their exposed locations. Caterpillars developing late in warm spots as pupa are then believed to represent individuals observed between December and February, before the first migrants return in March. 

So what do Red Admirals do in the winter? Well as a migratory species they have a broadly similar, if not as spectacular ecology as the Painted Lady. Individuals which emerge in late summer migrate south to the Mediterranean. Here they seek nettle beds which have recovered from the hot summer conditions which force the butterfly north to breed. These now lush winter nettles are used for breeding around November, before adults die. The resulting caterpillars slowly develop in the cooler conditions overwinter, with new adults emerging from March. These then move to higher altitudes (as in the Pyrenees and Alps) or north to breed in the spring, with many returning up to the eutrophic nettle paradise that is the British Isles.