Early in 2009 warmer weather (compared with 2007 and 2008) led to an expectation of a slight improvement in numbers of Chalkhill Blue (Polyommatus coridon) across the Chilterns and Berkshire Downs after two years of very slight population decline. Presumably the wet weather of the late summer, coinciding with the butterfly's flight period, was the reason that this was achieved at very few sites. Most colonies hosted further reduced numbers of this species. Records suggest that it failed to return to any recently abandoned sites or to colonise any new sites. Vagrancy away from the core sites was similarly less than in several recent years, if our records are any guide. The number of tetrads returning detail of sightings reduced from 73 (in 2008) to approx 60, though some of this fall relates to unvisited tetrads and to visited tetrads previously occupied but with no record in 2009, that are thought to be ones with regular vagrancy from strong nearby colonies in previous years, rather than with tiny colonies of their own.

Chalkhill Blue
Photo © Nick Bowles

With the exception of the weak satellite colony at Stepps Hill (part of the Ivinghoe Beacon, NT holding) the species was still recorded at all the visited sites with known, persistent colonies. So, hopefully, this reduction in overall numbers is temporary and a return to drier summers will see it recover numbers and possibly expand to colonise new areas.

A study at Pitstone Hill showed that the species had still not returned to the larger part of the hill, from which it was lost approximately 12 years ago, although it still breeds in one small gully on the hillside. The reason seems to be the inability of the once widespread Horseshoe vetch (Hippocrepis comosa) to re-establish after mysteriously disappearing from a large area.

Contact with three new landowners should help it persist on one site (a golf course) and assist in management aimed at consolidating its return to a second (where vagrant males had been an annual event until this summer). The third site may be seeded with Horseshoe vetch as, though it seems ideal in many ways, the sole foodplant is entirely absent.