The large Blue butterfly dispearred from Britain in the 1970's and in 1983 efforts were made to reintroduce it to the UK.
Anyway a survey at the National Trusts Collard Hill site in 2007 counted 354 adults beating the previous record of 300. Experts believe the warm spring helped the butterflies apparently it was the longest and earliest flight season since the reintroduction.
Apparently changes in pastoral grazing led to the initial decline. The large blue depends heavily on the red ant Myrmica sabuleti. The ants seem to thrive in short grass and the shift away from grazing leds to sites becoming over grown.
When the butterflies were reintroduced grazing was established at the sites. The large Blue was reintroduced to Collard Hill in June 2000 (12 females, 3 males) with 267 larvae in July.
In 2006 it was believed that the total UK population was up to 10,000.
Last years open day at Collard Hill was 24th June and so since I'm close to Collard Hill 21st/22nd I will have to take the camera....