Therfield Heath, Royston, Hertfordshire. Common land. Steep well-drained slopes on chalk.
Free public access, with a free car park.

This site has a strong colonies of Marbled Whites and Chalkhill Blues.
After a 30 year absence Dark Green Fritillaries were seen in 2012 by the transect walker and other observers.

Personal sightlings:

11 June 2011. Western end. 7 Small Heaths, 1 Common Blue, 10 Meadow Browns.

22 July 2012. Walking east from the top west corner of the hills. Western corner with the wood behind and to the south. Thin, poor grass. 17 Chalkhill Blues (15 male, 2 female), 1 Marbled White, 7 Meadow Browns.
The bank close to and north of the road to Therfield. Longer grass. 1 Small Skipper, 6 Chalk Hill Blues (5 male, 1 female), 6 Gatekeepers, 7 Meadow Browns, 1 Small Heath.
Along the ridge nearest to and south of the road. 11 Chalkhill Blues, 9 Marbled Whites, 4 Gatekeepers, 7 Meadow Browns, 1 Ringlet.
From the second gully south of the road to the 3 small tumuli on top of the down. 1 Large White, 9 Chalkhill Blues (2 female, rest male), 4 Marbled Whites, 17 Meadow Browns. The Chalkhill Blues gradualy thinned out as the grass got stronger towards the 3 small tumuli. 1 unidentified butterfly circled past flying fast like a Dark Green Fritillary, looked right size and colour.

26 July 2012. Walking east along the top of the heath from the Therfield road. Beyond the tumuli the numbers of Chalkhill Blues soon increased with less lush grass all the way to the eastern end. They were too numerous to count. Saw possible Fritillary again but not close enough to be sure.