On Friday 10 August, Tower Hamlets biodiversity officer John Archer was astonished to find a male Long-tailed Blue butterfly on his regular lunchtime visit to East India Dock Basin Nature Reserve. It was involved in an aerial battle with a newly-emerged Common Blue, which was clearly trying to establish a territory. This happened several times over about 40 minutes, before the Long-tailed Blue decided it had received enough aggression and flew into the treetops, never to be seen again. This left several very disappointed visitors who arrived from all over London during the afternoon to see the rare visitor. Only former Tower Habitats steering group member Bob Watts, who raced over from his regular lunchtime birdwatching at Tower Hamlets Cemetery Park, arrived in time to see it before it flew off.
Though common and widespread in southern Europe and throughout much of Africa and Asia, the Long-tailed Blue is a very rare migrant in Britain, averaging just one or two records per year. Records seem to have increased in the last few years, and the species can be expected to become more frequent here as the climate becomes warmer. However, it is unlikely to become permanently established in the near future because it is active throughout the year. All our resident butterflies have a dormant phase during the winter – some hibernate as adults, others as eggs, caterpillars or pupae.
This is probably the first one seen in London since 1990, when Long-tailed Blues bred at Gillespie Park in Islington, the first confirmed breeding of the species in Britain. When such a rare butterfly turns up, it is impossible to be sure of its origins. It is possible that it was released by someone who breeds butterflies, or it could have been accidentally imported with plants or vegetables. However, the time of year, and the prevailing fine weather with southerly winds, support it being a natural migrant from southern Europe – though perhaps it might have hitched a ride part of the way on a boat coming up the Thames.
Both photos of the Long-tailed Blue were taken at East India Dock Basin by John Archer.