By Mark Kinver
Science and nature reporter, BBC News
A record number of rare large blue butterflies were counted at a key breeding site during 2007.
A survey at Collard Hill, Somerset, counted 354 adults during 2007, beating the previous record of 300 in 2003.
Experts believe a warm spring helped the caterpillars at the National Trust-owned site develop quickly before the arrival of a very wet summer.
Efforts to re-introduce the species began in 1983 after it disappeared from the UK in the late 1970s.
"Despite the poor summer, 2007 was a remarkable year for the large blue at Collard Hill," explained Matthew Oates, nature conservation adviser for the National Trust.
"It saw record numbers of butterflies in flight and it was the earliest and longest flight season since its re-introduction."
Dr Martin Warren, chief executive of Butterfly Conservation, welcomed the survey's findings.
"This is marvellous news for one our most endangered species of butterfly," he said.
"With seven out of every 10 butterfly species in decline, Butterfly Conservation is delighted to be working with the National Trust to save this, and other species."
In the 1970s, scientists discovered that the reason why the large blue (Maculinea arion) became locally extinct was a result of changes to the way the rural landscape was managed.
The butterflies are dependent on the Myrmica sabuleti red ant (Image: Jeremy Thomas)
A team led by Jeremy Thomas, from the Centre for Ecology and Hydrology, Dorset, found that the survival of the butterflies was closely linked to a particular species of ant.
Professor Thomas observed that up to five species of red ants would "adopt" a large blue caterpillar, but the butterfly would survive in the nest of only one - the Myrmica sabuleti red ant.
But the decline of pastoral grazing saw a demise in the population of these ants, which in turn caused the large blue butterfly to disappear from the UK.
He found that the ants thrived in areas with short grass because sunlight was able to warm the soil, which suited this species.
Yet a shift away from grazing resulted in sites becoming overgrown, which caused the soils to cool.
As part of the reintroduction programme by the conservationists, grazing was re-established on the sites chosen for the butterflies.
Emerging early allowed the site's butterflies to breed before the rains arrived(Image: Dave Simcox)
Their efforts to manage the habitat paid dividends during the summer of 2006, when an estimated 10,000 of the creatures were recorded at sites across southern England.
"Generally, 2006 was a very good year for large blues in England," recalled Mr Oates.
"So, in theory, 2007 was set up very well by the previous year.
"But then we had this unprecedented spring drought in April which affected different large blue sites in different ways.
"One of the many threats facing large blues is spring droughts, and at the original large blue [reintroduction] site the ant populations were adversely hit by the dry April, so the large blue larvae had a very bad time as a result."
Mr Oates suggested that the site at Collard Hill was less susceptible to droughts than other sites, which meant that it was able to cope with the unseasonably dry weather.
He added that the butterflies at the Somerset site emerged in early June, allowing them to mate and lay their eggs before the heavy rains arrived in the second half of the month.
Despite the successful year, England's population of large blues has a history of "boom-and-bust" years, and remains listed as a priority species on the UK's Biodiversity Action Plan.