Tuesday, 19 August 2008

A shimmering carpet of blue

The chalk escarpments of southern England are a second home for me; their steep slopes providing vantage points from which to take in that ‘green and pleasant land’ triumphalised in Blake’s poem ‘And did those feet in ancient time’, better known as the hymn “Jerusalem’. I grew up here, in the verdant growth of the heavily wooded Surrey Weald, with the chalk downs rising to the south and framing my youthful years.

I return to the woods and downs fairly often, particularly so at this time of the year when the chalk escarpments have an added draw, alive as they are with a shimmering carpet of blue butterflies. Many of the remaining chalk grasslands around Winchester support good numbers of chalkland butterflies, some of which will have been on the wing much earlier in the year, but for sheer numbers it is the August mix of Chalkhill Blues and Silver-spotted Skippers that is most impressive. The steep slope at Old Winchester Hill, edging the Meon Valley, had a peak count of 1,000 plus Chalkhill Blues last year – a poor comparison though when compared with the 6,800 seen flying together back in 2006. The short turf of a chalk sward, rich in flowers and invertebrate life, is a good backdrop against which to watch the male blues as they drift back and forth across the turf in search of a receptive female. The skippers are less leisurely in their flight, whizzing around to nectar at flowers, challenge rivals and to seek out a mate. The short sward is important for the blues because these particular butterflies spend much of the year living within the nests of ants and the success of the ants themselves is heavily influenced by temperature. A hot, south facing-slope provides just the right amount of warmth for the ants but if the vegetation gets too tall, it shades out the ant nests and the required temperature cannot be maintained.

Of course we have our own band of chalk here in East Anglia, skirting the western fringes of Norfolk and cutting south through Cambridgeshire and Hertfordshire. I’ve written before of the Chalkhill Blues at Therfield Heath, just west of Royston, explaining how the small population at this site was once much bigger, drawing in collectors because of the incredible variation present within the butterflies themselves. However, these days the numbers here are insignificant when compared with the spectacle of the Hampshire downs and the hundreds or thousands of blues that can be seen on the wing together.

If you are ever in Hampshire during late July or early August make an effort to visit some chalk downland; you won’t be disappointed.

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