A late butterfly

The other afternoon, whilst walking home for lunch, I came across a red admiral butterfly, flying strongly above a high stone wall. This is my latest record for an active red admiral but perhaps it should not be that unexpected. The red admiral seems to be responding to global climate change, with very many more individuals seen on the wing in Britain now than was the case back in the 1970s. Some of these individuals are overwintering successfully as adults, with reports of territorial males and egg-laying taking place in the first three months of the year. This is particularly interesting because the red admiral is really a migrant visitor, breeding here in summer but thought unable to survive our winters. The first of the summer migrants reach us from the end of March, arriving from North Africa and the warmer Mediterranean islands. This means that those seen in January and February are likely to have overwintered here. Larger numbers of migrants arrive in late May or June, with these individuals originating from Spain and Portugal. The summer arrivals lay their eggs on nettles; the larvae that emerge making a simple tent by folding over the leaf and then holding it in place with silk. It is these youngsters that, having undergone the amazing transformation of metamorphosis, are seen on the wing as adults from August to October, when peak numbers occur.

From mid-August, perhaps triggered by shortening day-length, the strong-flying adults begin to move south. Large numbers often congregate in the extreme southern parts of England before crossing the English Channel. Yet, over recent years, increasing numbers have been seen on the wing later into the year to the extent that November records have become commonplace in some parts. Unlike certain other species, which as residents do overwinter here as adults, the red admirals on the wing so late into the winter usually perish. While peacocks, brimstones and small tortoiseshells choose sheltered sites for hibernation, the red admirals seem to settle on tree trunks (a site used for roosting at other times of the year). This would seem to make them more vulnerable to predators, parasites and the worst of the winter weather. Red admirals have also been reported overwintering as eggs, larvae and pupae and, in all cases, the development time of each particular stage is slowed. Once again, this may leave them open to predators and disease, leading to high losses and reducing the chances of successful overwintering.

No doubt things are changing, with more red admirals overwintering. However, since spring abundance is not correlated with the number seen late in the autumn, it seems that our increasing summer numbers result from migrants rather than those that try to overwinter.