Tuesday, 21 June 2011

Blues lifted by the heat

The legacy of a warm start to the year has seen the early emergence of many of our butterflies and moths, with several species on the wing a week or more ahead of usual. One of the species to benefit has been the Silver-studded Blue, a stunning little butterfly that (within Norfolk) is restricted to a few areas of suitable habitat in the northern half of the county.

In Britain, the Silver-studded Blue is primarily a heathland species, with just three populations in the west of the UK using other types of habitat. In parallel with the tremendous loss of lowland heath, this blue butterfly has undergone a substantial decline in both numbers and range. The national pattern of decline has been mirrored in Norfolk, where the natural history records held by Gresham’s School document the disappearance of former North Norfolk colonies to land reclamation and afforestation. Fortunately, the species has shown something of a revival over the last two decades, much of which has been due to the efforts of volunteers supporting the work of Butterfly Conservation.

Our Silver-studded Blue populations are centred on Kelling Heath in North Norfolk and Buxton Heath and Horsford Rifle Range just to the north of Norwich. In a good season the density of butterflies can be substantial, particularly if you make your visit some two to three weeks after the butterflies first emerge; a visit towards the end of June would be good this year. Males emerge before the females, a not uncommon behaviour in butterflies and something that is known as protrandry.

While many of our familiar butterflies are wanderers, with some actually moving into Britain from the Continent, the blues tend to be rather sedentary in their habits and the Silver-studded Blue is no exception. Most individuals move less than fifty metres over their lifetime, instead remaining within a patch of suitable heathland where they can nectar on heather, cross-leaved heath, bramble and bird’s-foot trefoil. Like most blues, Silver-studded Blues are dependent upon the presence of particular ant species (most notably Lasius niger and Lasius alienus), in whose nests the butterfly’s larvae develop. This means that adult densities are closely correlated with those of ant nests; as such, the best place to search for the butterflies is in areas of recently disturbed ground where the ants benefit from the warm conditions associated with a short sward structure.

Volunteers from the Norfolk branch of Butterfly Conservation are leading two weekday trips to look for Silver-studded Blues over the coming weeks. The first is Tuesday 28th June on Buxton Heath at 10am and the other is on Wednesday 6th July on Kelling Heath at 2pm. All are welcome and full details can be found at www.norfolk-butterflies.org.uk.

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