Tuesday, 21 August 2007

Chalk-hill Blues benefit from careful management

As I head southwest down the A11 and onto the A505 so I leave the acidic, sandy Breckland soils behind and cross onto the underlying chalk. Much of the chalk grassland has been lost to agriculture and the remnants are restricted to the steeper slopes of the chalk escarpment and to areas of favourable land-use, such as horseracing and golf. I am heading to the chalk grassland to search out the chalkhill blue, a butterfly that is on the wing in August. Instead of stopping near Newmarket to view colonies on the Devil’s Dyke and Fleam Dyke, I am travelling further afield to a colony on Therfield Heath, just to the west of Royston. My reason for visiting this particular colony lies in its history; Therfield famously attracted butterfly collectors from across Britain, each drawn by the lure of the varied colour forms of the butterfly to be found on the heath. Most of our butterflies are known to exhibit unusual colour forms from time to time, known as aberrations, but the chalkhill blue is noted for having more forms than is typical. The large population at Therfield Heath numbered many thousands and early last century the heath would have been inundated with collectors, each seeking that elusive aberration to add to their collection. One particular aberration, known as semi-syngrapha, in which the normally brown-coloured female has blue wings typical of the male, was the main target of the collectors.

Sadly, the pressure of collecting, coupled with changes in land management – notably the cessation of grazing by sheep, resulted in a dramatic decline in numbers and the population of chalkhill blues fell to dangerously low levels. These were restricted to the tiny pockets of suitable chalk grassland that remained, some only a few square metres in size. Fortunately, thanks to the efforts of local volunteers with an interest in the butterfly, the population was saved from extinction and since 1989 the number of chalkhill blues has increased. I wanted to see the results of this work and pay homage to the volunteers’ efforts by visiting the heath to photograph the butterfly.

One unusual aspect of the heath is the presence of a golf course, many fairways and greens of which are perched precipitously on the chalk escarpment. It is a large site and I thought that it might prove difficult to locate the discrete colonies. However, it did not take long to find the blues, the males actively quartering the vegetation in search of females that remained hidden below. These were delightful butterflies and seeing a dozen together made me wonder how it must have been to witness hundreds on the wing when the population was at its peak

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